Key strategies for enhancing the cycling stability and rate capacity of LiNi0.5Mn1.5O4 as high-voltage cathode materials for high power lithium-ion batteries

Journal of Power Sources 316 (2016) 85e105

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Review article

Key strategies for enhancing the cycling stability and rate capacity of LiNi0.5Mn1.5O4 as high-voltage cathode materials for high power lithium-ion batteries Ting-Feng Yi*, Jie Mei, Yan-Rong Zhu School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan, Anhui 243002, PR China

h i g h l i g h t s
Progress in understanding and modifying LNMO from various aspects is summarized.
Possible fading mechanisms of LNMO are discussed in details for the first time.
Key strategies for improving the cycling stability of LNMO is discussed.
Strategies in the development of LNMO-based batteries are discussed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 December 2015 Received in revised form 10 March 2016 Accepted 17 March 2016

Spinel LiNi0.5Mn1.5O4 (LNMO) is one of the most promising high voltage cathode materials for future application due to its advantages of large reversible capacity, high thermal stability, low cost, environmental friendliness, and high energy density. LNMO can provide 20% and 30% higher energy density than traditional cathode materials LiCoO2 and LiFePO4, respectively. Unfortunately, LNMO-based batteries with LiPF6-based carbonate electrolytes always suffer from severe capacity deterioration and poor thermostability because of the oxidization of organic carbonate solvents and decomposition of LiPF6, especially at elevated temperatures and water-containing environment. Hence, it is necessary to systematically and comprehensively summarize the progress in understanding and modifying LNMO cathode from various aspects. In this review, the structure, transport properties and different reported possible fading mechanisms of LNMO cathode are first discussed detailedly. And then, the major goal of this review is to highlight new progress in using proposed strategies to improve the cycling stability and rate capacity of LNMO-based batteries, including synthesis, control of special morphologies, element doping and surface coating etc., especially at elevated temperatures. Finally, an insight into the future research and further development of LNMO cathode is discussed. © 2016 Elsevier B.V. All rights reserved.

Keywords: Li-ion battery Cathode Spinel lithium nickel manganese oxide Fading mechanism Cycling stability

1. Introduction Recently, environmental issues are continuing to exert pressure on an already stretched world energy infrastructure. Significant progress has been made in the development of renewable energy technologies such as solar cells, rechargeable batteries, fuel cells and biofuels [1]. However, these energy sources have been marginalized with the increasing specific energy and power demands. Among various new energy storage technologies, Li-ion

* Corresponding author. E-mail address: [email protected] (T.-F. Yi). http://dx.doi.org/10.1016/j.jpowsour.2016.03.070 0378-7753/© 2016 Elsevier B.V. All rights reserved.

batteries (LIBs) have become the prime candidate due to the high voltage, high energy density and environmental friendliness [2]. In the past two decades, LIBs have dominated the portable electronic markets [3]. However, the power density of LIBs still needs to be further improved to fulfill the demand of the induststrial battery products although they have the highest energy density [4]. For LIBs, the main components are the electrode materials including anode and cathode. The commercialized anode used in LIBs is graphite. It's cheap and non-toxic [5]. In fact, cathode materials play a vital role in the determination of performance of the LIBs, and the cathode material is an important component of the LIBs, accounts for about 40% of the cost of battery. A vast series of Li-storage cathode materials has been explored in the past two decades,

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such as layered LiMO2 (M ¼ Co, Mn, Ni, or MnxNiyCoz) [6e13], layered Li-rich xLi2MnO3$(1-x)LiMO2 (M ¼ Co, Mn, or Ni), spinel LiMn2O4, spinel LiNi0.5Mn1.5O4 (LNMO), and olivine LiMPO4 (M ¼ Fe, Co, Mn, or Ni) [7]. So the exploration of the cathode materials attracts intensive attention. LiCoO2, first reported by Goodenough et al. [6], has been considered as one of the most important cathode materials owing to the high energy density and good cycling performance [7e10]. But the structural instability and capacity fading are serious. Even worse, it could cause severe safety issues in the practical application [11e13]. Li-rich materials usually have poor cycling stability and rate capability, which originate from the elimination of oxygen vacancies and phase transformation as well as the dissolution of transition metals, limiting their further development and application [14,15]. Despite based on good theoretical capacity and safe operating voltage, olive LiMPO4 is criticized due to the low electronic conductivity [16e19]. Fortunately, spinel lithium manganese oxide (LiMn2O4) as a cathode material for power LIBs has been paid much attention due to its low cost, low toxicity, and relatively high energy density [20e22]. However, poor rate performance in the 4-V region has often been observed owing to inferior structure stability [23e26]. Many previous works have proved that the electrochemical performance of LiMn2O4 can be improved by doping different cations and anions [27e29]. As we know, high voltage plateau can increase the total energy density of battery. Other cation-substituted spinel oxides LiM1.5Mn0.5O4 (M ¼ Co, Fe, Cr, and Cu) can also deliver capacity at about 5 V [30], but the only Ni substituted spinel LNMO is one of the most promising high-voltage materials for Li-ion battery application due to the acceptable cycling performance. Among the doped LiMn2O4 (LMO) materials, LNMO cathode received much attention on account of their improved cycling behavior and discharge voltage (4.7 V plateau) relative to the undoped spinel [30e32]. Obviously, the LNMO delivers a higher energy density (650 Wh kg1) of active material than that of LiCoO2 (518 Wh kg1), LiMn2O4 (400 Wh kg1), LiFePO4 (495 Wh kg1) and LiCo1/3Ni1/3Mn1/3O2 (576 Wh kg1) due to its high potential. It also has many advantages such as high operating voltage at 4.7 V(vs. Liþ/Li), high rate capability, no Jahn-Teller effect, low cost, good thermal stability, high electronic and lithium-ion conductivities [27e32], making it a promising material for practical use [31]. The main problem about LNMO is a possible corrosion reaction between the cathode surface and the electrolyte at the high voltage of 5 V, and then leads to a poor rate stability, especial at elevated temperatures. This article will sum up some important researches related the high voltage spinel oxides, including structure, electrolyte optimization, element doping, surface coating and control of special morphologies etc.

different crystallographic structures is the atomicity of Mn ions, and main Mn4þ and little Mn3þ ions exist in disordered LNMO with a space groups of Fd-3m. However, only Mn4þ ions present in the ordered LNMO with a space groups of P4332. Because of the existence of Mn3þ, the crystal with disordered space groups of Fd-3m has higher electronic conductivity [35]. According to the reports [36], the crystal with disordered space groups is single phase transition and the crystal with ordered space groups is two phase transition, but P4332 ordered phase reacts through a three-phase transition. Oxygen loss in the LNMO framework leads to the formation of Mn3þ ions in order to keep the electric neutrality, and the larger ionic radius of Mn3þ (0.645 Å) compared to Mn4þ (0.530 Å) results in a larger cell volume (Fig. S1) [37]. Hence, the crystal with disordered space groups have the better transmission path of electronic and Liþ [38]. Therefore, the disordered LNMO has better electrochemical performance than ordered one. The two structures can be achieved by calcining temperature control. When the calcining temperature is less than 700  C, the crystal structure is ordered. With the temperature increasing, the ordered structure translate to the disordered structure. If the material has an annealing process, the disordered structure will translate to the ordered structure [39,40]. The space difference can be determined with XRD technique, FT-IR spectroscopy and Raman spectra as given in Fig. 2 [41,42]. XRD patterns of LNMO with P4332 and Fd-3m space groups are quite similar, but there are two noteworthy features that differentiate the samples. There is a nickel-rich LixNi1xO rock salt impurity phase as evidenced by the weak reflections at 2q z 37, 43 , and 64 in LNMO with Fd-3m space group [43]. In LNMO with P4332 space group, it can be found a decrease of lattice parameter and symmetry caused by cation ordering, and weak superstructure reflections around 2q z 15 , 24 , 35 , 40 , 46 , 47, 57, and 75 can be found [44]. However, the structural difference between these two space groups is hardly to be clearly distinguished by X-ray diffraction because of the similar scattering factors of Ni and Mn [45]. Raman spectra and FT-IR spectroscopy have proved to be effective techniques in qualitatively resolving the cation ordering. Compared with LiMn2O4, the introduction of Ni2þ induces an increase of the number of vibration bands in the Raman spectra and FT-IR spectroscopy [46e48]. The irreversible representations of ordered and disordered spinel LNMO are given by Ref. [42]:

2. Structure and electrochemical performance of LiNi0.5Mn1.5O4

According to the Raman spectra (Fig. 2b), it can be found that there is more Raman peaks in the P4332 LNMO phase than that of Fd-3m one. However, the FTIR spectra are much less easy to discriminate between P4332 and Fd-3m phases (Fig. 2c). Compared with Fd-3m phase, two peaks at around 589 and 555 cm1 become more intense in the P4332 phase, and three new peaks at about 646, 464 and 430 cm1 are absent in Fd-3m structure. The peak at about 624 cm1 in Fd-3m phase are more intensive than those at 589 cm1, which is contrary to the P4332 phase [49].

2.1. Structure of Li Ni0.5Mn1.5O4 The crystallographic structures of LNMO has been reported in numerous works (see Ref. [33] for a review). LNMO spinel has two different crystallographic structures: the disordered space groups of Fd-3m and the ordered space groups of P4332. The former is facecentered spinel, in which the Ni and Mn, Li and O atoms are occupied in the 16d octahedral sites, 8a tetrahedral sites and 32e sites, respectively. In the meantime, Ni and Mn atoms are randomly occupied in the 16d sites (Fig. 1) [34]. For comparison, the latter is the simple cubic phase, in which the Ni, Mn, and Li atoms are occupied in the 4a, 12d, and 4c sites and O ions are occupied in the 8c and 24e sites (Fig. 1). In contrast with the former, Ni and Mn atoms are ordered regularly in the 16d sites. A difference of the

GFd3m ¼ Ag ðRamanÞ þ Eg ðRamanÞ þ 3F2g ðRamanÞ þ 4F1u ðIRÞ (1)

Gp4332 ¼ 6A1 ðRamanÞ þ 14Eg ðRamanÞ þ 20F1 ðIRÞ þ 22F2 ðRamanÞ

(2)

2.2. Charge-discharge performance of LiNi0.5Mn1.5O4 The charge curve of disordered material presents two voltage platform at 4.7 V, which is due to the redox reaction of the Ni2þ to Ni3þ and Ni3þ to Ni4þ, and near 4.0 V can be attributed to the Mn3þ to Mn4þ (Fig. 2d). While the charge curve of ordered material only

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Fig. 1. Structure of cation disordered (Fd3m) and ordered (P4332) spinel LNMO. Reproduced from Ref. [34] with permission from The Royal Society of Chemistry.

Fig. 2. Representative (a) X-ray patterns [41], (b) Raman spectra [42], (c) FT-IR spectroscopy [42] and (d) charge-discharge curves [41] of ordered and disordered LNMO samples. Reproduced from Ref. [41] with permission from The Royal Society of Chemistry. Reproduced from Ref. [42] with permission from Elsevier.

has a long flat plateau at 4.7 V. As similar with the charge curve, the discharge curve of disordered material presents two voltage platform and the order material only has one voltage platform. The ordered P4332 material has no 4.0 V voltage platform, indicating that there is no Mn3þ in the spinel lattice. Another marked difference is that the Ni plateau at 4.7 V is continuous with no obvious break between the Ni2þ/Ni3þ and Ni3þ/Ni4þ reactions, supporting the premise that a two-phase mechanism dominates rather than a solid-solution regime [50]. The excellent performance of LNMO cathode with high discharge voltage also promotes the investigations on full batteries, such as LNMO/Li4Ti5O12, LNMO/TiO2, LNMO/TiNb2O7, LNMO/mesocarbon microbead (MCMB), LiNi0.5Mn1.5O4/Si, and LNMO/Sn full batteries, and the more information can see the review [33].

2.3. Transport properties of LiNi0.5Mn1.5O4 The electrochemical performance of electrodes is influenced by the lithium ion insertion/extraction kinetics corresponding to the charge transfer reaction and lithium ion diffusion in the bulk of the material [51,52]. The poor electronic conductivity and lithium-ion diffusion ability usually limit the full capacity of electrode materials during cycling. Hence, information on the transport properties of LNMO is important and described in the following sections. The pristine LNMO compound usually exhibits a relatively high electronic conductivity (~104 S cm1) as determined by EIS measurement, which is comparable to that of other lithium manganese oxide such as LiMn2O4 (~104 S cm1) [53], but much higher than that of lithium metal phosphates such as Li3V2(PO4)3

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(~108 S cm1) [54], LiFePO4 (~1010 S cm1) [55] and LiMnPO4 (~1011 S cm1) [56] and slightly lower than LiCoO2 (~103 S cm1) [57]. The low electronic and ionic conductivities of LNMO will cause slow kinetics of charge and discharge, resulting in poor rate capacity and low capacity utilization. Various approaches, such as doping, surface coating and designing the special morphology, have been proved to be effective ways to increase the electronic and ionic conductivity of LNMO. Kunduraci et al. [58] found a 2.5 orders of magnitude difference between the lowest conductor in the ordered structure (P4332) and the highest conductor in the disordered (Fd3m) spinel structure. The electron hopping from increased content of Mn3þ to Mn4þ is explained as the basis of higher electronic conduction of disordered spinel than ordered spinel, which is absent of Mn3þ. Lee et al. [59] reported that LixMn1.5Ni0.5O4 with perfectly ordered P4332 phase exhibits only one two-phase region in entire range of 0 < x < 1, while uniformly disordered Fd-3m exhibits two pronounced two-phase regions of 0 < x < 0.5 and 0.5 < x < 1 with the possibility of more ground states at high lithiation depending on the existence of local deviations from the overall Ni/Mn ordering. In the LixMn1.5Ni0.5O4 with Fd-3m phase, only 8a tetrahedral sites were electrochemically involved when x was less than 1. The two voltage plateaus located at about 4.7 V were related to the transition between Ni2þ and Ni3þ, Ni3þ and Ni4þ. The further overstoichiometric Li intercalation results in a cubic-to-tetragonal phase transition when x reaches 3. When x is electrochemically pushed to about 4, the coexistence of a rock-salt structure with a layered component can be observed (Fig. 3) [60]. LixMn1.5Ni0.5O4 with ordered P4332 structure has two distinct paths for lithium migration. Lithium moves from one tetrahedral site to the next by migration through a vacant octahedral site [61], the vacant octahedral sites are divided into 4a(1/8, 1/8, 1/8) surrounded by three Ni and three Mn atoms and 12d (1/8, 5/8, 3/8) sites surrounded by one Ni and five Mn atoms. Migration of lithium between tetrahedral sites occurs through intermediate vacant octahedral sites. As shown in Fig. 3b, in path I, three Ni atoms change from 2þ to 4þ, then lead to an increase of electrostatic repulsion and activation barriers along this path of Liþ delithiation. The activation barriers in path II are obviously lower

than those in path I both in the lithium-rich phase and the vacancy-rich phase [62]. Several techniques including cyclic voltammetry (CV), electrochemical impedance spectroscopy(EIS), galvanostatic intermittent titration technique (GITT), and capacity intermittent titration technique (CITT) have been extensively used to study the diffusion kinetics of Liþ intercalation/deintercalation and to estimate the chemical diffusion coefficients of Liþ (DLi) in solid LNMO electrodes [63]. CV peak current shows a square root dependence on the sweep rate, and the relationship of the peak current and the sweep rate is as follows [64],

Ip ¼ 2:69  105 n3=2 AD1=2 CLi v1=2

(3)

where Ip is the peak current obtained from CV curves, n is the number of electron involved in the reaction, A is the surface area of the electrode, CLi is the lithium ion concentration in spinel compound and n is the potential sweep rate. The lithium ion diffusion coefficient also can be calculated by EIS according to the following equation [65]:

Zre ¼ Rct þ Rs þ su2 1

DLi ¼

R2 T 2 2 s2 2A2 n4 F 4 CLi

(4)

(5)

where R is the gas constant, T is the absolute temperature, F is the Faraday constant, and s is the Warburg factor. According the following formula used by PITT, the lithium ion diffusion coefficient can be calculated [66]:

DLi ¼

. h i2 It 1=2 lp1=2 DQ

(6)

where l is the characteristic diffusion length, and DQ is the total charge. According to the GITT analytical theory, the lithium diffusion coefficient of the active material can be calculated by the following equation [67]:

Fig. 3. (a) Schematic illustration of the transition from cubic, to tetragonal, to layered and rock-salt phases with Li insertion into the spinel structure of LNMO with Fd-3m structure [60]; (b) Schematic figure of two distinct paths for lithium migration in LNMO with P4332 structure. Blue arrows show Li diffusion path I, and the blue ball is the 4a site in the middle of path I. Dark scarlet arrows show the Li diffusion path II with dark scarlet balls at 12d sites in the middle of path II [62]. Reproduced from Ref. [60] with permission from American Chemical Society. Reproduced from Ref. [62] with permission from The Electrochemical Society. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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4



Vm I DLi ¼ p 0 FA

2 

 dE=dx 2 l2 pffiffi ; t < < DLi dE=d t

LiPF6 in the following reaction equations [87e89]:

(7)

where Vm is the molar volume, I0 is the applied current, l is the diffusion length, E is the Galvanic cell potential, and x is the deviation from the initial stoichiometry. It is found that the diffusion coefficient is closely related to the voltage states of charging and discharging, synthesis method, particle size and test methods. For example, the DLi of LNMO synthesized by a solegel method and firing at 850  C in air for 6 h obtained from CV by Yang et al. [68] is around 7.6  1011 cm2 s1. Our group [69] reported that the DLi of LNMO synthesized by EG-assisted oxalic acid co-precipitation and ammonium hydroxide coprecipitation method calculated by EIS are 2.03  1015 and 1.01  1015 cm2 s1, respectively. Kovacheva et al. [70] reported that the DLi of micro- (~1.25 mm) and nano-sized (~20 nm) particles between 4.6 and 4.8 V of LNMO are 1011e1013 and 1016e1015 cm2 s1, respectively. Ito et al. [71] reported that the DLi of LNMO prepared by spray drying, then re-annealing in O2 between 3 and 4.9 V is 1013e109 cm2 s1. 3. Failure mechanism of LiNi0.5Mn1.5O4 LiNi0.5Mn1.5O4 usually shows obvious capacity degradation at high temperature (>60  C) in full battery when the carbon-based materials are used as the negative electrode [72]. Currently, the capacity degradation mechanisms are attributed to structurerelated Mn dissolution [73,74], or structure-electrolyte-related reaction [75e77], etc. It has been reported that LiNi0.5Mn1.5O4 losses oxygen and disproportionates to a spinel and LiyNi1yO when it is heated above 650  C [78]. The formation of the LiyNi1yO impurity phase can be shown by the generalized reaction [79]:

LiMn1:5 Ni0:5 O4 /aLiy Ni1y O þ bLiMn1:5þx Ni0:5x O4 þ gO2 (8) where a, b and g define, respectively, the relative amounts of the LiyNi1yO, LiNi0.5xMn1.5þxO4, and O2 phase. Hence, Mn3þ exists in LiNi0.5Mn1.5O4 positive electrodes, and then it suffers severe Mn dissolution problem [80,81]. Pieczonka et al. [82] found that the amount of dissolved transition metals actually increases with the state-of-charge (SOC), That is to say, the dissolution is enhanced at the charged state, not the discharged state with generally low transition metal valences. Trace water impurity in the electrolyte can cause the liberation of acid HF through the decomposition of LiPF6, when LiPF6-based electrolyte was used. The chemical reactions were proposed as follows [83e85]:

LiF6 þ H2 O/LiF þ 2HF þ POF3

89

(9)

LiPF6 /LiF þ PF5

(13)

PF5 þ H2 O/2HF þ PF3 O

(14)

PF5 þ 2xe þ 2xLiþ /xLiF þ Lix PF5x

(15)

PF3 O þ 2xe þ 2xLiþ /xLiF þ Lix PF3x O

(16)

þ PF 6 þ 2e þ 3Li /3LiF þ PF3

(17)

HF will dissolve LiNi0.5Mn1.5O4 proposed as follows:

4HF þ 2LiNi0:5 Mn1:5 O4 /3Ni0:25 Mn0:75 O2 þ 0:25NiF2 þ 0:75MnF2 þ 2LiF þ 2H2 O

(18)

At elevated temperature, these undesirable processes are accelerated, which significantly limits the practical application of LNMO as a cathode material in the LIBs [90]. From these reactions, it can be found that acid is self-catalytic, and HF is regenerated by the reaction of water with LiPF6 [91]. Kim et al. also [92] proved that the capacity fading of the LiNi0.5Mn1.5O4/graphite full battery is due to the impact of Mn dissolution, and active Liþ loss in the full-cell system through continuous SEI formation (electrolyte reduction) prompted by Mn reduced on top of graphite surface. However, contrary to the conventional wisdom, Qiao et al. [93] also found that the performance failure of the LiNi0.5Mn1.5O4 is related to the formation Mn2þ determined by electrode-electrolyte surface reactions, instead of disproportional reactions. As shown in Fig. 4, it can be found that Mn2þ ions evolve disproportional reactions during the first charge process, and arrives at the maximum (about 60%) at the fully charged state on the side of the electrode facing separator. This indicates that Mn dissolution and electrolyte degradation are the two main reason of capacity degradation for LNMO based high-voltage electrodes. Pieczonka et al. [94] reported the Mn and Ni dissolution behaviors under various conditions, including state of charge (SOC), temperature, storage time, and crystal structure of LNMO in LNMO/ graphite full-cells. According to Fig. 4g, the self-discharge behavior of LNMO makes a decomposition of electrolyte, and the generated HF due to the decomposition of electrolyte can accelerate Mn and Ni dissolution from LNMO, and then various reaction products, such as LiF, MnF2, NiF2, and polymerized organic species, can be found on the surface of LNMO electrodes, which will increase battery-cell impedance. Especially, the Mn2þ ions can be reduced on the surfaces of graphite negative electrodes in full-cells by partly depleting active Liþ by the following reaction.

Mn2þ þ 2LiC6 /2Liþ þ Mn þ graphite

(19)

(10)

The reduced metallic Mn will further promote the loss of active Liþ through formation of thick SEI layers, and then leads to a significant capacity fading of LNMO/graphite full-cells.

2 þ HF PO2 F 2 þ H2 O/PO3 F

(11)

4. Synthesis

PO3 F2 þ H2 O/PO3 4 þ HF

(12)

POF3 þ

H2 O/PO2 F 2

þ HF

In addition, the reactive POF3 in the electrolyte reacts with carbonate solvents, such as EC, EMC, and DMC, to produce CO2 and OPF2ORF [86], and then destroys the SEI film during cycling. In addition, LiPF6 itself contains a small amount of HF during the manufacturing process [87], and the salt can easily react with water. The unavoidable ultra trace water in the electrolyte can react

On the basis of the above mentioned structure, the electrochemical performance of LiNi0.5Mn1.5O4 is greatly affected by synthesis methods and conditions. The synthesis methods mainly include solid-state reactions and wet chemical (solution synthetic) methods, and then lead to different structures, morphologies and sizes. The main synthesis methods and structures of LiNi0.5Mn1.5O4 are summarized as follows. The synthesis methods, precursors, synthesis conditions, and merits/shortcomings for each method as

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Fig. 4. Evolution of the chemical valence of Mn in LNMO electrode. (a) The electrodes with different SOC used for sXAS measurements are marked on the voltage profiles; Mn Ledge sXAS spectra (TEY) collected on the sides of electrodes facing separator (b) and current collector (c); (d), (e) and (f) Evolution of the Mn valence deduced from spectral simulations, with respect to the charge/discharge states [93]; (g) schematic diagram Mn and Ni dissolution in LNMO/graphite full-cell [94]. Reproduced from Ref. [93] with permission from Elsevier. Reproduced from Ref. [94] with permission from American Chemical Society.

well as the electrochemical performance are listed in Table S1.

4.1. Solid-state synthesis With two phases, obtaining single-LNMO is quite difficult [95]. Furthermore, the presence of impurities such as NiO or LixNi1xO deteriorates its cycling behavior [96e99]. Among all the methods, solid-state method is commonly used with a low cost. Fang et al. unitized LiCl$H2O, NiCl2$6H2O and MnCl2$4H2O as the reactants to synthesize LiNi0.5Mn1.5O4. The powders delivered a capacity of 110 mAh g1 even at 8 C [100]. Zheng et al. [101] prepared LiNi0.5Mn1.5O4 via a solid-state method. With a certain amount of disordered phase, the LiNi0.5Mn1.5O4 offered a discharge capability of 96 mAh g1 at 10 C and obtained a retention of 94.8% after 300 cycles. Zhu et al. [102] prepared LiNi0.5Mn1.5O4 by oxalic acidpretreated solid-state process. This material delivered an initial discharge capacity of 136.9 mAh g1 and capacity retention of 93.4% after 300 cycles at 0.3 C.

4.2. Co-precipitation synthesis It must be noted that LiNi0.5Mn1.5O4 synthesized via solid-state method has poor purity and large particle size. Thus various synthesis methods have been developed. The co-precipitation method has been reported by many groups [34,103e105]. Zhang et al. [34] prepared LNMO powders via a poly(ethylene glycol)-assisted coprecipitation method, and the synthesis route was illustrated in Fig. 5a. The PEG4000-assisted LNMO showed remarkably cycling performance with discharge capacity of over 120 mAh g1 at 40 C and a capacity retention of 89% after 150 cycles at 5 C. In addition, an organic coprecipitation process was applied to obtain LNMO [104]. The sample had stable structure and rate performance during the cycling. It delivered a discharge capacity of more than 130 mAh g1 at 0.2 C. 4.3. Sol-gel synthesis Sol-gel method and solution combustion synthesis have also

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91

Fig. 5. (a) The illustration of the co-precipitation route from Refs. [34]; (b) the schematic diagram for the formation of LiNi0.5Mn1.5O4 [105]. Reproduced from Refs. [34,105] with permission from The Royal Society of Chemistry.

been applied [105,106]. Cubic MnCO3 was used as template to prepare LiNi0.5Mn1.5O4, as shown in Fig. 5b [105]. It demonstrated that the sample showed excellent cyclic performance with a capacity of 78.1% after 3000 cycles at 10 C and 83.2% after 500 cycles under 5 C even at 55  C. Liu et al. [106] synthesized single phase LiNi0.5Mn1.5O4 with calcination temperature of 700  C for 30 min. The results indicated that the product was single phase LiNi0.5Mn1.5O4 with Fd-3m space group. It exhibited excellent rate properties and large lithium diffusion coefficient. The LiNi0.5Mn1.5O4 delivered capacities of 130 and 113 mAh g1 at 1 C and 10 C, respectively. After 100 cycles, the retentions were 97.5% and 99.7%, respectively.

4.4. Other synthesis Molten salt synthesis was considered as a simple technique to synthesize complex oxides with the desired composition in a lowmelting point flux [107]. Wen et al. [108] prepared the spherical LiNi0.5Mn1.5O4 cathode materials by molten salt synthesis (MSS) method, and the discharge capacity was 129 mAh g1 in the first cycle and 127 mAh g1 after 50 cycles under an optimal synthesis condition for 12 h at 800  C. Kim et al. [109] synthesized wellordered high crystalline LiNi0.5Mn1.5O4 spinel with Fd-3m space groups by a molten salt method, and the LiNi0.5Mn1.5O4 powders synthesized at 900  C for 3 h delivered an initial discharge capacity of 139 mAh g1 with excellent capacity retention rate more than 99% after 50 cycles. The powder prepared by emulsion drying method usually yields a homogeneous powder precursor in which the cations are intermixed homogeneously at atomic scale from the emulsion state [110]. Myung et al. [111] prepared the nano LiNi0.5Mn1.5O4 with Fd3m cubic spinel structure by the emulsion drying method, and it showed good cyclability fired at 750  C for 24 h. The composite carbonate process can combine the solegel and solid-state reactions, and share many advantages of the above two synthetic methods. The main advantage of the carbonate process is obtaining a well-developed stoichiometric LiNi0.5Mn1.5O4 material with a higher surface area at a lower synthesis temperature. Lee et al. [112] prepared the well-developed LiNi0.5Mn1.5O4 by the composite carbonate process, which contained many spherical particles of about 3e4 mm, made up of small nano-sized particles (50e100 nm), and showed an improved the cycling performance. Compared with the methods mentioned above, the hydrothermal route can improve crystallinity and reduce the particle sizes

[113]. Xue et al. [114] synthesized the LiNi0.5Mn1.5O4 via ethanolassisted hydrothermal method, and the LiNi0.5Mn1.5O4 showed excellent electrochemical performance with discharge capacity of 81.7 even at 20 C. Moreover, the sample delivered remarkable longterm cyclability with lower impurities and Mn3þ content. After 100 cycles at 5 C, the discharge capacity was about 102.1 mAh g1. It also had good performance with a capacity retention of 82% after 200 cycles at 55  C.

5. Control of special morphology Nanostructuring of the electrode materials can enhance the mass diffusion length [115e120]. Nanostructures, including fibers, nanorods and nanoplates have been reported as excellent materials with superior rate performance [121e123]. Arun and coworkers [121] synthesized the spinel LiNi0.5Mn1.5O4 fibers via spinneret electrospinning method. The cycling performance of Li/LNMO fiber cells cycled between 3.5 and 5 V is shown in Fig. S2. The cell delivers an initial reversible capacity of about 118 mA h g1 with good cycleability and renders about 93% of the initial reversible capacity after 50 cycles. The one-dimensional nano fibers architecture can keep the effective contact areas large and fully realize the advantage of active materials at nanometer scale. Hence, a onedimensional LNMO fiber is one of the most favorable structures as cathode materials for high-performance LIBs. LNMO porous nanorods (LNMO PNR) were prepared via a morphology-inheritance route, as given in Fig. S3a [122]. The inset of Fig. S3b shows that the prepared LNMO rods are composed of interconnected nanosized subunits with highly porous structure. It delivered a capacity of 109 mAh g1 at 20 C and capacity retention of 91% after 500 cycles at 5 C. The one-dimensional nanorods can provide short transport path along the confined radial dimension. The porous nanorods framework can not only allow for efficient contact between active material and electrolyte but also accommodate better the strains related to the structural transformation upon repeated intercalation/de intercalation of lithium ions. Yang et al. [123] prepared LiNi0.5Mn1.5O4 nanoplates using twostep method composed of a hydrothermal and solid-state method. Fig. S4(a) shows a possible mechanism for the formation of nanoflake-stacked LiNi0.5Mn1.5O4. From Fig. S4(b), it can be observed that the LiNi0.5Mn1.5O4 powder is composed of nanoplate assemblies with approximate dimensions of 80 nm  100 nm. Fig. S4(c) indicates that LiNi0.5Mn1.5O4 nanoplates delivered a specific discharge capacity of 134.2 mA h g1 at 15 C and

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120.9 mA h g1 even at 40 C, which is 95.2% and 85.8% of the 140.9 mA h g1 obtained at 1 C, respectively. The improved rate performance of LiNi0.5Mn1.5O4 nanoplate can be attributed to the shortened diffusion path of Li-ion and the nanoplate-stack structure. The two-dimensional nanoplates can provide short ion/electron diffusion path and also be beneficial for accommodating the strain induced by the severe volume variations during the repeated intercalation/de intercalation of lithium ions. Zhu et al. [124,125] developed a novel composite coprecipitation to prepare spherical hierarchical LiNi0.5Mn1.5O4 (NLNMO) with high electrochemical performance, as shown in Fig. S5. The capacity and retentions of N-LNMO are 136.0 mAh g1 (96.3%), 128.6 mAh g1 (94.4%), and 113.9 mAh g1 (91.1%) under 0.3 C, 1 C and 3 C rates after 200 cycles, respectively. However, LiNi0.5Mn1.5O4 synthesized by traditional co-precipitation method (T-LNMO) suffers a serious fading, and the capacity retention under 0.3 C is only 95.2 mAh g1 (72.5%) after 200 cycles. The large secondary spherical hierarchical particle is composed of smaller nano-scale or submicron primary particle. The coarse surface resulting of the hierarchical structure is beneficial for the infusion of electrolyte into the cathode and then improve the electrochemical performance of LNMO. Chen et al. [126] prepared pseudo-sphere-like chamfered polyhedral LNMO spinel (LNMO-COh) and octahedral structure LNMO (LNMO-Oh) by a polymer auxiliary method under air and O2 atmosphere, respectively, as shown in Fig. S6. Both LNMO-COh and LNMO-Oh delivered high discharge capacity and cycling stability at 25  C. The capacity retentions of LNMO-COh were 90.0% after 200 cycles at 55  C. However, the discharge capacity of LNMO-Oh dropped quickly as the cycle number increased when the testing temperature was increased to 55  C. The LNMO-Oh with octahedral structure shows only {111} crystal faces, but the LNMO-COh shows new {110} crystal orientations. The {110} surface facets in LNMO spinel are also favorable for lithium ion diffusion compared to {111} surface facets. These indicate that the chamfered polyhedral LNMO has a far superior rate capacity compared to the octahedral structure, attributed to the existence of new {110} crystal orientations, which would be more beneficial for lithium ion diffusion. Hence, the unique structure can be considered as an effective way to improve the kinetic performance and electrochemical performance, such as specific capacity, specific energy, specific power, high rate performance and cycle life. 6. Doping Cations or anions doping has been considered to be an effective way to improve the electronic and ionic conductivity of LNMO materials. Without exception, the structure and electrochemical properties of LNMO can also be affected by the substitution of other ions. To the best of our knowledge, the cationic doping elements mainly include Naþ, Mg2þ, Cu2þ, Zn2þ, Al3þ, Cr3þ, Co3þ, Fe3þ, Sm3þ, Rh3þ, Ga3þ, Ru4þ, Zr4þ, Ti4þ, Nb5þ, V5þ, Mo6þ, W6þ, etc., while the reported anions mainly consist of F and S2 etc. However, single ion doping may not effectively improve the rate capacity or cycling stability of spinel LNMO, especially at elevated temperatures. Hence, dual or multiple ion-doped LNMO materials have also been developed to enhance electrochemical performances. In the following sections, we mainly focus on the cycling performances of ion-doped LNMO in half cells, and the typical synthesis method and electrochemical performance of the doped LNMO are given in Table S2. 6.1. Monovalent and divalent cations doping As is known, the sodium is abundant, cheap and environmental

friendly. Thus it is a promising dopant. In addition, Na-doping usually leads to a decrease of conduction band, thereby causing a band gap reduction, indicating that the electronic conductivity of electrode material can be increased by replacing Li atom with Na atom [127]. Hence, Na doping has been considered as an effective way to improve the electrochemical performance of LiMn2O4 [128], LiFePO4 [129], Li3V2(PO4)3 positive-electrode materials [130] and Li4Ti5O12 negative-electrode material [131]. Wang et al. [132] synthesized the Na-substituted Li1xNaxNi0.5Mn1.5O4 by a solid-state method. They found that the Na-doping destroyed the ordering of Ni and Mn ions, so the proportion of Fd-3m spinel increased with the rise of Na content. As shown in Fig. 6a, the crystal lattice parameters increase gradually with increasing of the doped Na, and 1, 3 and 5% Na doped LNMO electrodes show higher rate capabilities and cycling stability than those of pristine LNMO electrode at evaluated temperature. The improved electrochemical performance can be attributed to the better charge transfer ability, relieve the ohmic polarization and electrochemical polarization of materials and improved lithium ion diffusion coefficient due to the Na doping. As for magnesium, it is cheap and abundant. Moreover, it has been reported that the Mg substitution not only lowers the polarization but also improves the overall insertion kinetics of LNMO by increasing the electronic conductivity [133e145]. In the studies conducted by Lafont [136] and Wagemaker [137], the doping of Mg improved the electronic conductivity of LiMg0.05Ni0.45Mn1.5O4 and LiMg0.1Ni0.4Mn1.5O4, and the host structure was also stabilized. Shiu et al. [138] prepared the Mg-doped LiNi0.5xMgxMn1.5O4 (x ¼ 0, 0.02, 0.05, and 0.10) with Fd-3m space group by a spray pyrolysis method. As shown in Fig. 6b, Mg-doping increases the lattice constant of LNMO, and restrains the formation of LixNi1xO. Mgdoped LiNi0.5Mn1.5O4 electrodes show low capacity fading rate at 5 C rate when cycling at 60  C. Cu-doping often increases electronic conductivity of electrode material due to the special outer electronic arrangement of Cu2þ. Cu-doping spinel materials have been studied by a few research group [139e141]. Yang et al. [142] has proved that Cu could lower the Liþ diffusion barrier potentially. Sha et al. [143] prepared Cudoped LiNi0.5xCuxMn1.5O4 (x ¼ 0, 0.03, 0.05, and 0.08) with P4332 space group via a sol-gel method. As shown in Fig. 6c, the crystal lattice parameter increases gradually with increasing of the doped Cu, and the cycling test demonstrates that the LiNi0.45Cu0.05Mn1.5O4 obtained the best performance at evaluated temperature. As given in Fig. 6(c), it achieves 124.5 mAh g1 at 5 C with the retention of 97.7% after 150 cycles. The high performance of Cusubstituted LNMO can be ascribed to the faster lithium ion mobility, lower polarization and better structural stability than those of pristine one. Later, Milewska et al. [144] studied LiNi0.5yCuyMn1.5O4(y ¼ 0, 0.02, and 0.05) at different annealing temperatures. Results show that the LiNi0.48Cu0.05Mn1.5O4 annealed at 800  C shows the best cycling performance and highest diffusion coefficient of Liþ. It is well known that the zinc is abundant and less expensive than the many transition metals, so Zn substituted electrode material is expected to be cathode material with lower cost than other transition metals substituted LNMO. Based on this, Yang et al. [145] investigated the effect of Zn substitution for Ni on the structure and cycling properties of LNMO with Fd-3m space group prepared via the sol-gel method. As given in Fig. S7, the lattice parameter increases gradually with increasing of the doped Zn. The capacity retention of 95% and rate capacity for LiZn0.08Ni0.42Mn1.5O4 are obviously higher than those of pristine LNMO after 100 cycles at 0.5 C rate. In addition, LiZn0.08Ni0.42Mn1.5O4 also exhibits a better cycling performance than that of pristine LNMO at 60  C.

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Fig. 6. (a) Cycling performance of the as prepared Li1xNaxNi0.5Mn1.5O4 samples discharged at 5 C rate at 55  C [132]; (b) cycling performance of the as prepared LiNi0.5xMgxMn1.5O4 cycled at 5 C rate at 60  C [138]; (c) cycling performance of LiNi0.5Mn1.5O4 and LiNi0.45Cu0.05Mn1.5O4 at 5 C rate at 55  C [143]; (d) cyclic performance of LiNi0.5xAl2xMn1.5xO4 (0  2x  0.60) charged-discharged at 1C rate at 55  C [150]; (e) cyclic performance of LiNi0.5Mn1.5O4 and LiMn1.5Ni0.42Ga0.08O4 charged/discharged with a constant current of C/6 at 55  C [154]; (f) rate performance of LiNi0.5SmxMn1.5xO4 (x ¼ 0, 0.01, 0.03, 0.05) at different rates [158]; (g) rate performance of LiNi0.5Mn1.5O4 and Fedoped LiNi0.5Mn1.5O4 at different rates [160]; (h) cycling performance of pristine and Ru-doped LiNi0.5Mn1.5O4 charged-discharged at 10 C rate [193]; Reproduced from Refs. [132,138,143,150,154,158,193] with permission from Elsevier. Reproduced from Ref. [160] with permission from American Chemical Society.

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6.2. Trivalent cation doping Al3þ-substituted LNMO can be expected to be one the most promising cathode material because Al is abundant, nontoxic, less expensive and lighter than other transition metal dopants. Hence, great efforts have been paid to verify the enhancement of high temperature performances by Al doping on the 16d Mn or Ni sites of LNMO spinel [146,147]. It has been demonstrated that the Al doping can improve the electrical conductivity of LNMO [148e151]. Zhong et al. [150] synthesized the LiNi0.5xAl2xMn1.5xO4 (0  2 x  1.0) via a thermopolymerization method. The substitution of Al in LiNi0.5Mn1.5O4 increased the disordering degree of Mn/ Ni with the spinel structure transferred from P4332 to Fd-3m. As shown in Fig. 6d, Al concentration significantly enhances the rate capability and cycling stability of LNMO. The cycle performance at elevated temperature is also dramatically improved due to the Al doping. The capacity retention after 100 cycles increases from 74.7% for LNMO to over 95% for the LiNi0.5xAl2xMn1.5xO4 samples (0.025  2x  0.6). Gallium is also known as a good candidate material for dopants because it improves the structural stability of LiNiO2 [152] and LiV3O8 [153] cathode materials. Shin et al. [154] synthesized LNMO and LiMn1.5Ni0.42Ga0.08O4 powders with P4332 and Fd-3m space groups by a hydroxide precursor method. However, the degree of ordering of the Mn4þ and Ni2þ ions in the LiMn1.5Ni0.42Ga0.08O4 spinel structure is suppressed. The LiMn1.5Ni0.42Ga0.08O4 sample shows high capacity and superior cyclability at 55  C compared to pristine LNMO (Fig. 6e) due to the elimination of the LixNi1xO impurity phase, stabilization of the disordered spinel structure, and a robust cathodeeelectrolyte interface provided by the decoration of the surface with Ga3þ ions. The Sm3þ dopant has been reported to be effective due to the electronic distribution [155e157]. Mo and coworkers prepared LiNi0.5SmxMn1.5xO4 (x ¼ 0,0.01,0.03 and 0.05) by gelatin-assisted solid-state method [158]. The FT-IR spectra demonstrates that Sm-doped spinels possess the more disordered Fd-3m space groups. As plotted in Fig. 6f, all of the doped LNMO materials show significantly improved rate performance, and LiNi0.5Sm0.01Mn1.49O4 shows the highest rate capacity among all samples due to the complex effects of the more disordered Fd-3m space group and the better electronic conductivity by the import of Sm3þ. It has been reported that the electrochemical performance of LNMO spinel could be improved by iron ion (Fe3þ) doping [159,160]. The presence of Fe in the tetrahedral sites of the structure stabilizes the solid during extended cycling. Liu and Manthiram [160] reported the structure and electrochemical performance of the Fe-doped LNMO samples synthesized by a hydroxide precursor method. Fe substitution stabilizes the cation disordered structure of LNMO. The cycling performance of Fe-doped LNMO samples as given in Fig. 6g indicates that all Fe-substituted samples deliver higher capacities, capacity retentions and cycling stability than those of pristine one. The LiMn1.5Ni0.42Fe0.08O4 composition offers a combination of high capacity and excellent cyclability among all samples due to the stabilization of the cation-disordered structure and the much reduced lattice parameter differences. The structure and electrochemical performance of LNMO can be affected by the doping of Co [161,162]. It has been reported that the Co3þ replacement of Ni2þ and Mn4þ enhanced the rate and cycling performance with higher structural stability and electronic conductivity of LNMO [163,164]. Jang et al. [165] prepared the LiNi0.5xCo2xMn1.5xO4 (x ¼ 0.0e0.075) by a co-precipitation method. The doped LNMO shows better electrochemical properties due to the increasing electrical conductivity. It has been reported that Ni and Cr can be employed to substitute Mn in LiMn2O4 to improve the electrochemical properties due

to the high oxygen affinity of Cr3þ ions, providing structural stability during cycling [166]. Our work [167] showed that Cr3þ doping could suppress the formation of LixNi1xO or NiO impurity phase and improved the 5 V capacity and cyclability. Doping with chromium is a new suitable strategy to improve the electrochemical properties of LNMO, even at 55  C [168]. However, the substitution of chromium ions in LiMn2xCrxO4 results in a conversion of Cr3þ to Cr6þ for x > 0.2 and in the formation of LiCrO2 impurity for x > 0.8 [169]. Hence, the doped Cr content should be optimized. It may originate from no Jahn-Teller distortion retaining with the substitution [170e173]. Liu et al. [174] reported that the discharge capacities of LiNi0.4Cr0.1Mn1.5O4 were 141 and 125 mAh g1 cycled in 3.5e5.0 V at 0.5 and 1 C rates, respectively. Another component LiNi0.4Cr0.15Mn1.45O4 has been prepared via a sol-gel method with tartaric acid-assisted [175]. It was found that the formation of impurity in the final ones was suppressed and the cation ordering was increased with the doping. Wang et al. [175] reported that the LiNi0.4Cr0.15Mn1.45O4 delivered an inital discharge capacity of 143.9 mAh g1, while the pristine one only had 136.6 mAh g1. After 40 cycles, they maintained 139.7 and 127.7 mAh g1 respectively. Moreover, Wang's group [176] studied a series of LiCr2yNi0.5yMn1.5yO4 (0  y  0.15). They found LiCr0.2Ni0.4Mn1.4O4 showed the best cycling performance. It exhibited a discharge capacity of 143 mAh g1 at 1 C with good retention of 96.5% after 50 cycles. In addition, some researchers focus on the chromium doping [177,178]. LiCrxNi0.5xMn1.5xO4(x ¼ 0.00, 0.01, 0.03 and 0.05) cathodes were fabricated [178]. It demonstrated that the discharge capacity and the retention of LNMO were enhanced, and the oxygen deficiency was reduced, which resulted in better structural stability. Now LiCr0.2Ni0.4Mn1.4O4 has been proved to be a promising composition because it has higher voltage plateau and cycling stability than those of pristine LNMO [179e184]. The reported synthesis methods include sol-gel [179,180], combustion [181e183] and hydroxide precursor, and so on. Nie et al. [184] prepared the LiCr0.2Ni0.4Mn1.4O4 cathode by one-step and two-step polymerization methods. The particle prepared via a two-step process shows lower polarization and impedance, and it obtains an inital discharge capacity of 145.5 mAh g1 at 1 C rate, and still retains 140 mAh g1 after 100 cycles. Aklalouch et al. [181] studied the role of the particle size on the electrochemical properties at 25 and 55  C of the LiCr0.2Ni0.4Mn1.4O4 spinel prepared by a sucrose aided combustion method at temperatures from 700 to 1100  C. As shown in Fig. S8, the heat treatment temperature obviously affects the particle size of LiCr0.2Ni0.4Mn1.4O4, and the capacity loss by cycle notably decreases on increasing the heating temperature. There is a high variation of the capacity loss with the heating temperature when cycling at 55  C. These reveal that the cycling stability of the LiCr0.2Ni0.4Mn1.4O4 electrodes depends on the thermal treatment and the cycling temperature. Zirconium (Zr) also has similar effects like other dopants. Oh et al. [173] found that the structure of the Zr doped materials was ordered spinel and Cr doped spinel was disordered. 6.3. Tetravalent cation doping It has been reported that Ti4þ doping can improve the rate capacity of the pristine LNMO due to the TieO chemical bond being stronger than NieO, and then the Ti doping can increase its structural and chemical stabilities [185e187]. Alcantara et al. [186] reported that the doping of small amounts of Ti improved the electrochemical performance whereas a deterioration of the reversible capacity was observed for large amounts of Ti. Jin et al. [187] reported the carbon-coated LiNi0.4Ti0.1Mn1.5O4 exhibited more excellent cycling stability, higher discharge capacity and coulombic efficiency compared to the LNMO and LiNi0.4Ti0.1Mn1.5O4

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samples at 55  C. Kim et al. [188] reported the structure and electrochemical properties of LiNi0.5Mn1.5xTixO4 (0.2  x  0.4) by using half-cells paired with lithium metal, and full-cells paired with either graphite or Li4Ti5O12 negative electrodes. LiNi0.5Mn1.3Ti0.2O4 consists of both disordered (Fd-3m) and ordered (P4332) spinel phases, but LiNi0.5Mn1.5xTixO4 (x ¼ 0, 0.3, 0.35) electrodes are fully disordered (Fig. S9a). The electrochemical performance shows that LiNi0.5Mn1.2Ti0.3O4/graphite full-cell delivers the higher capacity than that of LNMO/graphite full-cell at each cycle at 45  C with strong loss of capacity, and LiNi0.5Mn1.2Ti0.3O4 and LiNi0.5Mn1.15Ti0.35O4/Li4Ti5O12 full-cells show higher discharge capacities and cycling stabilities than those of LNMO/Li4Ti5O12 full-cell at 45  C (Fig. S9b and c). However, the LiNi0.5Mn1.5xTixO4 (x ¼ 0, 0.2, 0.3, 0.35) Li4Ti5O12 full-cells show the noticeable capacity fading at 45  C because a severe electrolyte decomposition problem still exists in the full-cell system at the elevated temperature. Fig. S9d reveals that Ti-substituted LiNi0.5Mn1.5xTixO4/Li half cells have lower self-discharge capacities than that of LNMO/Li cell, indicating that the Ti-substituted electrodes oxidize the electrolyte less than €weling [189] et al. also reported Tithe pristine electrode does. Ho doped LNMO/graphite cells experience a lower, but still strong loss of capacity due to the loss of active lithium during cycling. In comparison with other metals, the 4d transition metals such as Ru favor wider conduction bands [190e193]. Wang et al. [192] found there was a new hopping pathway in Ru-doped LNMO as NieOeRueO, and thus electron transfer could be much easier due to large delocalized Ru4þ ions. In particular, Li1.1Ni0.35Ru0.05Mn1.5O4 and LiNi0.4Ru0.05Mn1.5O4 show better rate performance than that of pristine LNMO [193], and deliver discharge capacities of 108 and 117 mAh g1 at 10 C rate, as shown in Fig. 6h. The improved rate capacity is due to the improvement in electronic conductivity, which can support faster charge transportation at high current rates and is useful to prevent the pronounced pile-up of Liþ ions and undesired Mn3þ ions on surfaces of particles during high-rate discharge. Recently, Rh-doping has been demonstrated to be effective to enhance the electrochemical properties of LiCo1xNixO2 [194,195] and LiFePO4 [196]. Wu et al. [197] synthesized the Rh-doped LNMO by a sol-gel method. LiNi0.48Rh0.02Mn1.5O4 exhibits the best rate performance among all samples, and it obtains an initial discharge capacity of 92.4 mAh g1 even at 3000 mA g1. Moreover, it maintains a retention of 83.6% after 300 cycles under 150 mA g1 at 55  C, which is attributed to the suppression of electrolyte destruction by Rh-doping. 6.4. Pentavalent and sexivalent cation doping It has been proved that V5þ-doped spinel Li4Ti5O12 delivers higher discharge capacity and capacity retention than that of undoped spinel [198e201]. Then the substitution of vanadium in the Li sites of LNMO has been considered to improve the electrochemical performance. Kim et al. [202] reported the structure and electrochemical performance of V-doped LiNi0.5Mn1.5O4 spinel at elevated temperature conditions prepared by a scalable sol-gel technique mediated by adipic acid. LNMO and Li0.995V0.005Ni0.5Mn1.5O4 deliver discharge capacities of about 138 and 142 mAh g1, respectively. In contrast to LNMO, the cycling stability of V-doped sample at elevated temperature is significantly improved due to the improvement in the electronic conductivity profiles of said phase. As we know, the charge-discharge capacity of LNMO is from the reversible redox reactions between bivalent nickel ion and tetravalent nickel ion. Hence, the amount of Ni2þ determines the capacity of LNMO. According to our previous work [203], the Mn element has a supporting role of the spinel framework in the spinel

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lithium manganese oxide. Based on this, we design Nb and Modoped LNMO spinels, and the chemical formula are LiMn1.425Ni0.525Nb0.05O4, LiMn1.425Ni0.4Nb0.1O4, LiMn1.4Ni0.55Mo0.05O4 and LiMn1.425Ni0.5Mo0.05O4 [203,204]. In fact, some reports have been proved that Nb5þor Mo6þ-doped spinel Li4Ti5O12 delivers higher discharge capacity and capacity retention than that of undoped spinel [205e207]. Our group [204] reported the fast rate performance of Nb-doped LNMO materials prepared by solid-state method. Nb-doped LNMO materials show super cycle stability even discharged at 5 C rate. LiMn1.425Ni0.525Nb0.05O4 material presents the highest capacity at 5 C rate among all samples due to the increased the electronic conductivity and enhanced migration ability of lithium ion. Our group [203] synthesized Mo-doped LNMO via a sol-gel process. The electrochemical tests showed that the electrode polarization was reduced, which suggested that the Mo-doped LNMO had faster Li insertion/extraction kinetics during the cycling. In addition, the LiNi0.55Mo0.05Mn1.4O4 electrode showed the smallest particle size and the best rate capability and reversibility. W6þ-doped LiWxNi0.5Mn1.5xO4 (x ¼ 0.00e0.10) are synthesized via a sol-gel method by Prabakar et al. [208] They found that the incorporation of W into LNMO could alter the structure. Furthermore, the length of LieO band was increased and the distances of NieO and MneO were decreased. Thus, better capacity and higher capacity retention can be obtained. 6.5. Anion doping Beside those metal ions, anion doping can also result in excellent rate performance. A common dopant occupied in the anion site is fluorine. The reason is that a small amount of fluorine substitution for oxygen can reduce Ni and Mn dissolution from HF attack, enhancing the electrochemical properties and thermal stability [209]. The reversible capacity of high voltage LNMO spinel material was improved by fluorine doping [209e211]. Du et al. [211] prepared LiNi0.5Mn1.5O4xFx (0.05  x  0.2) materials by sol-gel and post-annealing treatment method, and a stable cycling performance was obtained when the fluorine amount x was higher than 0.1, but the specific capacity was decreased due to more stable finestructure arising from fluorine doping. Sun et al. [212] synthesized the LiNi0.5Mn1.5O4xSx (x ¼ 0, 0.05) compounds by co-precipitation using the metal carbonate (Ni0.5Mn1.5)CO3 as a precursor. The sulfur-doped spinel LNMO displays excellent capacity retention and rate capability in the 3-V region due to the rough morphology of the primary particles with smaller particle size. In addition, several reports indicate that multi-ion doping can more effectively improve the rate capacity or cycling stability of spinel LNMO. Leon et al. [213] reported the electrochemical performance of Ti and Fe doped LNMO spinel. The material containing 0.10Fe þ 0.10Ti shows one phase, but the material containing 0.05Fe þ 0.05Ti shows a two phase mechanism of lithium extraction. LiFe0.1Ti0.1Ni0.45Mn1.35O4 exhibits the best capacity retention due to the single phase mechanism combined with structural stabilization by Ti. Liu and coworkers [214] demonstrated that LiNi0.45Cu0.05Mn1.45Al0.05O4 exhibited the better cycling performance, delivering a capacity retention of 96% after 100 cycles at 1 C. Besides, Sha et al. [215] have prepared the LiNi0.475Al0.01Cr0.04Mn1.475O3.95F0.05 via a sol-gel method, and its discharge capacity at 0.2 C and 20 C were 136.6 and 66.4 mAh g1, respectively. 7. Surface coatings Surface coating has been considered as one effective and controllable approach to stabilize the electrode/electrolyte interface, and reduce the side reactions [216,217]. The coating materials mainly include various carbon materials, metals, oxides,

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phosphates, lithium compounds, polymers. The surface treatment of spinel LNMO could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the electrode dissolution during cycling test. We mainly concentrate on examples of surface coated spinel LNMO for the improvement of rate performance of cycling performance at elevated temperatures, and the typical synthesis method and electrochemical performance of the coated LNMO are shown in Table S3. 7.1. Carbon coating Carbon materials usually include graphene, fullerenes, nanotubes and graphite. Graphene, a single layer of carbon atoms bonded together in a honeycomb crystal lattice, is the fundamental structure in the carbon world. As shown in Fig. 7, graphene can be made into 0 D fullerenes, rolled into 1D nanotubes and stacked into 3D graphite [218]. Carbon coating can effectively increase the conductivity of the intrinsically insulated materials and the absorbing ability against the organic molecules, and protect the electrode from direct contact with electrolyte, and thus results in good rate capabilities. When the most common LiPF6-based electrolyte was used, the dominant lithium salt LiPF6 was sensitive to a trace amount of moisture. Trace water impurity in the electrolyte would cause the liberation of acid HF through the decomposition of LiPF6, and cause the dissolution of the transition metals and erode the surface of active materials (Fig. 7a), and then lead to capacity serious capacity loss. The carbon coating layer with high chemical and electrochemical stability as a layer of protection can retard the degradation of active material during cycling(Fig. 7b) [219]. With this advantage mentioned above, carbon coating has been demonstrated as an effective way to improve the electrochemical performance of the other electrode materials, such as phosphate materials [220e222], silicate materials [223e225], LiMO2 (M ¼ Co, Ni, Mn) [226e228], LiMn2O4 [229,230], Li4Ti5O12 [231e234] etc. Zhang et al. [235] synthesized the carbon modified sample by solgel method. The LNMO was partially covered by carbon particles, which presented an average size of about 70 nm. The composite exhibited enhanced rate performance with a capacity of 111 mAh g1, while the LNMO only obtained 70 mAh g1 at 5 C rate. The effect of carbon coating with different contents on the physical

and rate properties of LNMO was discusses by Yang et al. [236]. It demonstrated that the carbon coating could greatly improve the capacity and cycling stability of LNMO without degrading the spinel structure. The sample modified with 1 wt% carbon shows the best performance among all samples. Wang et al. [237] prepared carbon coated LNMO/C composite at 800  C without using reducing gas or introducing additional low temperature treatments using polyethylene glycol-400 (PEG 400) as carbon source(Fig. S10). Even at 10 C charge-discharge rate, LNMO/C delivers a highest capacity of 87 mAh g1 during cycling, and retains 64 mAh g1 (71% capacity retention) at the 500th cycle, which are obviously higher pristine LNMO. To further improve the energy density of LNMO, free-standing LNMO/carbon nanofiber (CNF) electrode has been reported by Fang et al. [238]. Though this approach, the total weight was reduced and the conductivity was enhanced. With such benefits, the LNMO/carbon nanofiber electrode showed excellent current rate capability even in 20 C rates as shown in Fig. S11. The porous CNF network facilitates electrolyte infiltration in addition to electron transfer and enhances the conductivity of LNMO electrode. Carbon nanotubes (CNTs), which can provide high conductivity and light-weight network yet, can access the electrons and Li-ions effectively when combined with the electrode [239e241]. Moreover, CNTs have been used with varying successes as additive for the electrode materials [242]. The combination of LNMO and multiwall carbon nanotubes were reported by Fang et al. [243]. Obviously, the LNMO/MWCNT electrodes apparently performed much better than the conventional electrodes, especially at large current rates. the CNT30 and CNT20 samples can maintain a capacity of 107.9 mAh/g and 84.4 mAh/g, respectively, while capacity from the conventional electrodes almost decreased to zero at 20 C charge-discharge rate (Fig. S12). Another allotrope of carbon, graphite also attracts intensive attention. It has been reported that graphene and graphene oxides can enhance the rate capability and cycling stability in Li-ion batteries and lithium sulfur batteries [244e251]. These materials have been demonstrated to be a stable wrapping layer in the cycling. Fang et al. [252] prepared graphene-oxide-coated LNMO, and found that the coated graphene-oxide layer was about 5 nm. The graphene-oxide-coated LNMO showed superior cycling performance compared with pristine LNMO due to the enhanced

Fig. 7. Schematic illustration showing the long-term degradation of active material in air and in electrolyte (a) without and (b) with carbon coating [219]; (c) Graphene, 0 D fullerenes, 1D nanotubes and 3D graphite [218]. Reproduced from Ref. [218] with permission from American Chemical Society. Reproduced from Ref. [219] with permission from The Royal Society of Chemistry.

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conductivity and reduced side reactions between the electrode and electrolyte. For further research, Manoco et al. [253] investigated the effect of different carbons on the rate performance of LNMO electrode. In the synthesis, partially reduced graphene oxide(pRGO) added to C65 or SP was adopted to prepare LNMO composite. Both LNMO/pRGO-C65 and LNMO/pRGO-SP show higher rate capacities and cycling stabilities than those of LNMO/C65(or SP) electrodes. 7.2. Noble metal coating In the case of propylene carbonate(PC)-based electrolytes, efficient passivating SEI film cannot form with the decomposition of PC. Hence, the intercalation of lithium is avoided. Instead, solvent molecules intercalate into graphite and the electrolyte also pass through the poor film at the same time, and thus cause the exfoliation of graphene sheets [254]. Moreover, the carbon atoms at the edge are still active. Consequently, the performance of graphite in PC-based electrolytes is not so good. Metal coating is a good choice to improve the electrode performance [255e257]. Xiong et al. [258] reported the structure and electrochemical performance of Ag coated LiNi0.49Mn1.49Y0.02O4 cathode prepared by electroless plating method. The Ag-coated sample shows superior cycleability and rate capability even at a high current density of 10 C at 55  C because of the improved electronic conductivity, reduced electrochemical polarization and increased the lithium ion diffusion coefficient. Arrebola et al. [259] investigated the electrochemical behavior of Au-coated LNMO powers with two different coating methods. One strategy, which involved treatment with HAuCl4 in HCOH, resulted in poorer rate performance. The other involved evaporating Au on the spinel, and it demonstrated that the capacity of low rates increased compared to the bare spinel. Though the Au or Ag-coated LNMO shows excellent electrochemical performance, the high cost limits further commercial application. Apparently the noble metal coating is not an attractive choices used in commercial lithium-ion batteries due to the high cost. 7.3. Metal oxide coating CuO, as anode material for the development of advanced LIBs, offers many favorable properties such as low cost, high theoretical capacity and eco-sustainability [260]. Verrelli et al. [261] reported the CuO/LNMO composite operated excellent efficiency and rate performance even at high rate. The LNMO powder has been prepared by Li et al. [262] through co-precipitation. The LNMO electrodes with excellent performance were modified with 1, 3 and 5 wt% CuO, respectively. The test demonstrated that 3 wt% Cuomodified sample showed the superior electrochemical performance. As shown in Fig. S13a, the pristine LNMO showed no modified layer on the surface, while the 3 wt% coated sample had uniform and amorphous modification layer with thickness of about 3 nm. The pristine and the coated ones show similar discharge capacity at low rates, as given in Fig. S13b. However, the 3 wt% coated sample delivers noticeably higher discharge capability than the others. Even at 10 C, it still exhibits 98.7 mAh g1. Quite stable compounds such as ZnO can enhance the electrochemical behavior of LNMO [263e267]. The presence of ZnO can reduce the solubility of LNMO, thus the amounts of dissolved Mn and Ni decrease [164,267]. Sun et al. [266] prepared pristine and ZnO-coated LNMO samples via sol-gel method, and found that the capacity retention of the coated LNMO nearly maintains 100% after 50 cycles, whereas the uncoated one retains no more than 10% after 30 cycles. A comparative study of LNMO coated by MgO and ZnO shows that both the ZnO and MgO coatings help to improve the stability of LNMO. However, MgO was found to be more effective than ZnO in this regard [268]. Alva et al. [269] reported the LNMO

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electrode materials coated by Mg-based coatings using two different heat treatment temperatures, 500 and 800  C, and named as BL, Mg500, and Mg 800, respectively. They found that Mg2þ was introduced mainly as an inhomogeneous MgO coating in the sample treated at 500  C, and into the spinel lattice at the subsurface of the particles at 800  C. Rate discharge capability tests at 50  C (Fig. S13c) were conducted, and the results showed that Mg800 displayed the best rate capability when going from C/10 to 5C, with retention of 62%, followed by BL then Mg500, with 48% and 42%, respectively. The reason is due to the incorporation of Mg into the spinel structure, thus improving the charge transfer kinetics. It has been reported that ultrathin Al2O3 coating layer could significantly enhance the stability and safety of the cathode materials including LiCoO2 [270], Li[Ni,Co,Mn]O2 [271], and Li-rich materials (Li[Li,Ni,Mn,Co]O2) [272] and anode including silicon [273], and metal oxides [274]. In recent years, Al2O3 coating deposited on LNMO electrode has been reported [275e278]. Kim et al. [277] investigated the effect of depositing ultrathin Al2O3 coatings on the electrochemical performance of LNMO. It demonstrated that the coulombic efficiency was significantly enhanced and the rate capability was also suppressed dramatically. Huang et al. [278] prepared Al2O3 coated LNMO via a novel carbamide-assistant hydrothermal process followed by a heat treatment. A homogeneous Al2O3 layer with a thickness of 20 nm can be seen on the surface of 1 at.% Al2O3-coated LNMO particles (Fig. S13d). Compared with pristine LNMO, Al2O3-coated LNMO exhibits remarkably improved cycling performances at 55  C because Al2O3 coating layer can restrain side-reactions. RuO2 is also a good choice used as an electrical contact material because it is excellent electronic conductor [279]. It has been reported that the RuO2 modification can improve the electrochemical properties of LiFePO4 [280], Li1.2Mn0.54Co0.13Ni0.13O2 [281] and Li3V2(PO4)3 [282] materials. The pristine and RuO2 modified LNMO samples were prepared by sol-gel method [283]. The amounts of RuO2 in the materials were determined to be 1.6, 2.0 and 3.0 wt%. In all samples, the 2 wt% RuO2 coated one showed superior rate performance. The lattice fringes with interplannar distance of 2.25 Å corresponded to (220) planes of RuO2 performed in HRTEM as shown in Fig. S13e. The rate performance (Fig. S13e) show that 2.0 wt% modified LNMO exhibits the best rate capability in all samples because the modification decreases the polarization of the electrode, prevents the side reactions of the electrode and the electrolyte, and improves the charge transfer reactions at the electrode interface. Silica has been widely used in LIBs. Fumed silica has been proved to be effective in trapping impurities when added to organic electrolytes [284e286]. It has been reported that the LNMO can be modified with commercialized fumed silica [287]. Shin et al. [288] demonstrated a facile method to improve the cycling performance of LNMO with SiO2-coating. The formed protective layer could reduce the dissolution of transition metals and suppress the irreversible decomposition of the electrolyte, especially at high voltages. As shown in Fig. S13f, pristine LNMO suffers from severe capacity fading after 50 cycles, but the modified one exhibits improved capacity retention at 55  C because the composite polymer layer containing SiO2 particles are formed on the surface of LNMO materials, which can effectively protect the active LNMO materials from HF attack, and results in suppression of Mn and Ni dissolution into the electrolyte. ZrO2 coated LNMO also has a similar effect, which can limit the surface side reactions [289]. It has been reported that vanadium oxides can react with LiOH/ Li2CO3 impurities and trace Li ions in the bulk layer when annealed at high temperatures. The formation of lithium ion intercalated compounds can purify LiNO2-based surface from physical adsorption of residue Li2O/LiOH to enhance the cycle performance at a

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high voltage [290]. Wang et al. [291] reported the V2O5-coated LNMO materials synthesized via a wet-coating method. The V2O5 coating layer in 5% V2O5- LNMO is about 3 nm (Fig. S13g). At 55  C, 5% V2O5-LNMO exhibits a capacity of 15% higher than pristine LNMO at fifth cycle, and the coated one shows a higher capacity retention than that of pristine LNMO after 100 cycles. They believe that V2O5 coating layer can form a stable barrier between LNMO and electrolyte at elevated temperature to prevent the side reactions between them, and then enhance the electrode hightemperature performance. Other metal oxides, such as La0.7Sr0.3MnO3, ZnAl2O4 and YBa2Cu3O7 (YBCO), seem to be promising as a coating material due to the high electronic conductivity in a wide temperature range. Zhao et al. [292] reported La0.7Sr0.3MnO3 modified LNMO exhibits remarkably enhanced electrochemical reversibility and stability at elevated temperature due to the reduced surface and chargetransfer resistances and increased lithium diffusion rate. Lee et al. [293] reported the ZnAl2O4-coated LNMO cathode exhibited the significant capacity retention even after storing at elevated temperatures (60  C) due to the suppressed side reaction between the cathode and the electrolyte. Lin et al. [294] reported LNMO@YBCO composite fabricated by a simple solegel method. Comparing to the pristine LNMO, LNMO@YBCO obviously demonstrates superior capacity cyclability at high temperature (Fig. S13h) because the YBCO layer on the LNMO surface efficiently suppress the surface reactivity between the charged electrode and the electrolyte during electrochemical cycling.

lithium polyacrylate (PAALi) coating to improve the electrochemical performance of LNMO. As shown in Fig. 8c, LNMO@1% PAALi sample exhibits a higher discharge capacity than that of other samples, and all coated samples show higher discharge capacity than LNMO at 12 C rate because PAALi promotes the formation of valid SEI film, being favor of the compatibility between electrode and electrolyte. Li2Oe2B2O3 glass was used as coating layer to improve the electrochemical cycling of cathode materials due to good ionic conductivity, such as LiMn2O4-based and LiNi1/3Co1/ 3Mn1/3O2-based electrodes [307e309]. Chae et al. [310] designed a Li2Oe2B2O3-glass-coated LNMO (GC-LNMO) cathode to enhance the thermal stability of LNMO-based electrode. From the HR-TEM image shown in Fig. 8d, it can be seen that a uniformly dispersed about 5-nm-thick layer of Li2Oe2B2O3 glass is homogeneously coated onto the surface of the LNMO particles, and GC-LNMO material delivers an initial specific capacity of 106.9 mAh g1 after 50 cycles at 60  C, corresponding to about 87% of its initial specific capacity. However, pristine LNMO only shows a capacity of 86.5 mAh g1 after 50 cycles at 60  C, corresponding to ~66% of its initial specific capacity. Other lithium compounds, such as LiAlO2, also were used to modify LNMO-based electrode because it is a fast ion conductor. Cheng et al. [311] reported that 1 mol.% LiAlO2surface modified LiMn1.58Ni0.42O4 samples exhibit excellent rate capability and cyclability compared to that of the bare one, especial at high temperature.

7.4. Lithium compound coating

Metal fluorides coating layer can suppress HF corrosion to active materials during cycling, and then can improve cycling stability of lithium-ion battery [312,313]. Li et al. [314] found that AlF3-coating improved the discharge capacity, cycling capacity retention, and rate capability of LNMO because the coating layer suppressed the electrolyte decomposition, and reduced the interface impedance. Wu et al. [315] synthesized AlF3-coated LNMO via a sol-gel method. As shown in Fig. 8(e), an appropriate amount of AlF3 coating layer can not only improve the cycling stability but also enhance the thermal stability. But excessive amount hinders the transportation of Li-ions, resulting in evident capacity decay. Huang et al. [316] reported the electrochemical performance of GaF3-coated LNMO, and the 0.5 wt% GaF3-coated LNMO exhibited an obviously better cycle life than the bare sample at 60  C, delivering a discharge capacity of 120.4 mAh g1 after 300 cycles. The reason is that the GaF3 layer increases the electronic conductivity of the LNMO, and effectively suppresses the reaction between the active material and the electrolytes. Recently, conducting polymers have been considered as potential additive to improve the cycling stability and rate performance of cathode materials for lithium-ion battery, such as such as Li3V2(PO4)3 [317], LiFePO4 [318] and LiCoO2 [319] etc. Among various conducting polymers, polypyrrole (PPy) conductive polymer has a high electrical conductivity (a few tenths of S cm1), and can be easily produced with the desired morphology by chemical reactions [320]. Gao et al. synthesized the PPy-coated LNMO (LNMO) composites with a thickness of around 3 nm (Fig. 8f) [321]. The cycling performances at elevated temperature (55  C) exhibits that all coated samples show the higher discharge capacity and capacity retention than those of bare LNMO. 5 wt% PPy coatedLNMO shows the best electrochemical performance among all samples. After 100 cycles, it still retains a reversible capacity of 105.2 mAh g1, corresponding to the capacity retention of 91%. According to the inset of Fig. 8f, the improved electrochemical performance of LNMO/PPy can be ascribed to the improvement of conductivity, suppression of the dissolution of transition metals and reducement of electrolyte decomposition. Cho et al. [322]

The lithium compound coatings have been used to enhance the electrochemical performances of LNMO at elevated temperatures or high rate performances, mainly include Li3PO4, Li4P2O7, LiAlO2, lithium polyacrylate (PAALi), Li2Oe2B2O3 glass, etc. Li3PO4 is known to be a fast solid lithium ionic conductor [295], and Li3PO4 coating has been used to improve the electrochemical performance of LiMn2O4 [296], LiCoO2 [297], LiFePO4 [298] cathode materials. Kobayashi and coworkers adopted Li3PO4 coating on LNMO to prevent the degradation of the solid electrolyte [299]. Konishi et al. [300] reported the effect of Li3PO4 coating on the LNMO epitaxial thin film electrodes synthesized on SrTiO3 substrates by pulsed laser deposition (PLD) with a thickness of 1e4 nm. They found that Li3PO4 coating affected the manganese valence near the electrode surface and improved the cycling characteristics. To build a stable protective layer of Li3PO4, Chong et al. [301] prepared the coated LNMO sample via a solid-state reaction. From Fig. 8(a), a coating layer about 5e6 nm thick in the composite can be found. The Li3PO4-coated LNMO showed better cycling properties compared with the noncoated one. At 0.5 C after 650 cycles, the coated sample retains 80% of the initial capacity while the uncoated almost faded completely after 345 cycles. Another lithium phosphates, Li4P2O7 is considered to be compound in LiePeO ternary phase diagram [302] and has been reported to provide a fast Li-ion transport pathway in LiFePO4 [303,304]. Chong's group [305] prepared Li4P2O7-stabilized LNMO by solid-state method following with calcination in different temperatures. The sample sintered at 760  C estimated a crystalline size of about 527 nm (Fig. 8b). LNMO/Li4P2O7 shows more stable cycling ability than the pristine LNMO because Li4P2O7 can build artificial SEI layer or solid electrolyte to protect the interface between the electrode and the electrolyte. After 893 cycles, LNMO/Li4P2O7 show a capacity of 92 mAh g1 with a retention rate of 74.3%. However, LNMO only delivers a capacity of 89.7 mAh g1 (70% of the initial capacity) after 252 cycles (Fig. 8b). Zhang et al. [306] designed a facile new strategy by using

7.5. Other compounds coating

Fig. 8. (a) cycling performance of pristine and Li3PO4-coated LNMO at 0.5 C rate [301]; (b) cycling performance of pristine and Li4P2O7-coated LNMO at 0.5 C rate (Inset is TEM image of Li4P2O7-coated LNMO synthesized at 760  C for 200 h) [305]; (c) Rate capability of pristine and coated LNMO with different amount of PAALi (Inset is TEM image of the LNMO @1%PAALi) [306]; (d) cycling performance of pristine and GC-coated LNMO at 60  C and 1 C rate (Inset is TEM image of GC-coated LNMO) [310]; (e) Cycling performances of the pristine and surface-modified LMNO electrodes at 0.1 C (Inset is FESEM image of 2 wt% AlF3-coated LMNO) [315]; (f) cycling performance of pristine and PPy coated LNMO at 1.0 C and 55  C (Insets are schematic illustration of how the PPy layer acts as a conductive and protective layer to suppress the dissolution of Mn and decomposition of electrolyte at elevated temperature, and TEM image of LNMOe5 wt% PPy) [321]; (g) cycling performance of pristine and PI-LNMO at 1C charge/discharge rate and 55  C (Inset is TEM photograph of PI-LNMO) [322]; (h) cycling performance of pristine and AZO-LNMO charged at 0.1 C and discharged at 5 C at 50  C (Inset is TEM image of AZO-LNMO) [326]. Reproduced from Ref. [301] by permission of Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced from Refs. [305,306,310,315,326] with permission from Elsevier. Reproduced from Refs. [321,322] with permission from The Royal Society of Chemistry.

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developed a facile approach to modify LNMO cathode by nanoarchitectured polyimide (PI) gel polymer electrolyte (GPE) coating with a thickness of approximately 10 nm (Fig. 8g). The cycling performance at 1 C charge/discharge rate and 55  C (Fig. 8g) exhibits that PI-LNMO presents a significantly higher cycling performance than that of pristine LNMO because the PI wrapping layer prevents the direct contact between LNMO and violent liquid electrolyte. Kim et al. [323] also reported an improved electrochemical properties of PI-coated LNMO at elevated temperature condition. The 0.3 wt% PI coated LNMO phase shows an excellent cycleability with capacity retention of >90% at 55  C. It has been reported that Al-doped ZnO (AZO) exhibits a high electronic conductivity [324,325]. Hence, Sun et al. [326] designed AZO-coated LNMO cathode with a thickness of approximately 1e2 nm prepared by solegel method (Fig. 8h). The cycling performance of pristine and AZO-LNMO charged at 0.1 C and discharged at 5 C at 50  C show that AZO-coated LNMO electrode shows the best cycling stability even at a high rate at high temperature. After 50 cycles, the corresponding capacities fade to 35.1, 17.1 and 115.0 mAh g1. The improved performance of AZO-LNMO is ascribed to the decreased charge transfer resistance and increased electrical conductivity. To avoid the interfacial side reactions between the solid electrolyte and LNMO at high voltages, Hu et al. [327] designed the ZrP coated LNMO with different contents by a hydrothermal process, as illustrated in Fig. S14a. It can be clearly seen that ZrP (2 wt%) has been well layer-by-layer coated on the LMNO surface, with a thickness less than 10 nm. 2 and 4 wt% ZrP coated LNMO show excellent cycle stability without obvious capacity fading, with 94.6% and 94.2% of the initial discharge capacities after 200 cycles (Fig. S14b), which is higher than those of bare LNMO. The reason is that ZrP layer prevents or suppresses the chemical reactions between cathode materials and non-aqueous electrolytes. Yang et al. [328] designed lattice doping and surface coating to improve the electrochemical performance of LNMO. They prepared Li0.1B0.967PO4 (LBPO)-coated LiNi0.45Cr0.1Mn1.45O4 with a thickness of about 5 nm by a soft combustion reaction followed by calcination illustrated in Fig. S15a. Cycling performance (Fig. S15b) shows that bare LNMO, Cr-LNMO and LBPO-Cr-LNMO deliver reversible capacity of about 101.1, 115.9 and 125.1 mAh g1 with 74.5%, 85.4% and 91.3% capacity retention over 400 cycles, respectively. They confirm that the LBPO coating layer is key to better performance. Most of the time, the role of surface coating layer is to inhibit HF attack from the LiPF6-based electrolyte, suppress transition metal dissolution, reduce charge transfer resistance, increase the electronic conductivity by coating metal or conducting polymers, and improve structural stability of the spinels. Most of surface coating decrease the initial discharge specific capacity, but the cyclability and rate capability, especially at elevated temperatures, can be significantly enhanced. It is a reasonable and wise choice to improve the electrochemical performance of LNMO by surface coating. 7.6. Electrode material coating The motivation for coating LNMO by other electrode material is to achieve a more balanced performance compared to what is possible with any individual compound. LiFePO4 coating can improve the rate capability and thermal stability of LNMO due to is high thermal stability in the charged state, environmental compatibility and low cost. LiCoO2 coating can improve the kinetics performance of LNMO because LiCoO2 show fast kinetics, as can be observed from a less pronounced electrode polarization at higher C-rates. Surface modification with LiFePO4 has remarkable improvement in the electrochemical properties of LiCoPO4 [329],

LiNi0.5Co0.2Mn0.3O2 [330] and LiCoO2 [331] etc. Jang et al. [332] found that the LiFePO4 modified LNMO displayed exceptional capacity retention even with marginal reduction in the initial. Liu and coworkers [333] prepared LiFePO4-coated LNMO via sol-gel method with mechano-fusion dry process. The particle size of LNMO was about 200 nm, while the thickness of the surface coating was 5 mm. At 1 C between 3.0 and 4.9 V, the bare achieved only 61.5% of the initial capacity after 100 cycles, while the LFP-coated one retained 74.5% after 140 cycles (Fig. S16a). LNMO has been considered as a competitive coating media to modify spinel LiMn2O4 because it possesses good structural stability, compatible spinel structure with the targeting LiMn2O4 and fast Liþ transfer character [334,335]. Qiu et al. [336] adopted a novel wet chemical method to prepare the LiMn2O4@LNMO (LMO@LNMO). In 3.0e4.3 V, the LMO@LNMO with the mass ratio of 9:1 (mLMO:mLNMO) and sintering temperature of 800  C obtained an initial discharge capacity of 100 mAh g1 at 55  C, and much improved cycling performance with capacity retention of about 81.9% after 400 cycles (Fig. S16b). LiCoO2 was used as surface coating material because it has a higher electric conductivity (102 S cm1) [337] than that of LNMO (105 S cm1). Co3O4 has a similar cubic spinel structure with LNMO [338], which provides a feasibility for using Co3O4 as coating media to prevent the side reaction between the LNMO material and the electrolyte. Qiao et al. [339] improved the electrochemical performance of LNMO by the surface modification used LiCoO2/ Co3O4 composite. From the HRTEM image in Fig. S16c, it can be found that the lattice fringes with the d-spacing values correspond to the (104) plane of LiCoO2 and the (311) plane of Co3O4, respectively, indicating that both LiCoO2 and Co3O4 are successfully coated on the surface of LNMO particles. The cycling performance (Fig. S16c) of all samples at 5 C shows that LiCoO2/Co3O4 modification dramatically improves the cycle stability of LNMO at high rate. Except for the suppression of the dissolution of LNMO, the improved cyclability of coated LNMO was also ascribed to the acceleration of the electron transfer and suppression of the side reactions by the LiCoO2/Co3O4 surface modification. Lithium titanates (Li2TiO3 and Li4Ti5O12) have threedimensional lithium-ion diffusion path and outstanding structural stability and thermodynamic stability in an organic electrolyte, and they are usually used to stabilize the structure of high-capacity cathode materials as a coating layer [340,341]. Deng and coworkers [342] modified LNMO with a coating of Li2TiO3 nanolayer. The HRTEM image shown in Fig. S16d reveals that the d-spacing of the coating layer is 0.480 nm corresponding to the d-spacing of the (002) plane of monoclinic Li2TiO3 with a thickness of 8 nm. The cyclability at 1 C rate and at 55  C shows that LNMO@Li2TiO3 (3, 5%) samples show higher discharge capacity and better capacity retention after 50 cycles at 55  C than the bare one. Our group [343] applied Li4Ti5O12 as an additive to improve the rate performance of LNMO. It demonstrated that the electrochemical stability and reversibility was improved as the coating can protect the surface of the electrode from HF in the electrolyte. Our group [344] also found that Li4Ti5O12 coating can improve the capacity retention and coulomb efficiency of Cr-doped LNMO spinel (LiMn1.4Cr0.2Ni0.4O4) because the coated Li4Ti5O12 layer suppressed the formation of passivation on the surface of the cathode, and then prevented the electrolyte decomposition and the Mn ion dissolution. 8. Summary and outlook The rapid development of Li-ion power batteries makes high demand for long-life and high energy density cathode materials. Combined with the merits of environmental friendliness and low cost, the spinel LNMO material has been demonstrated to be one of

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the most promising cathode material in practical energy application due to the high voltage (around 4.7 V vs. Li/Liþ). The main problems LNMO cathode face is the dissolution of transition metal (Mn and Ni) due to the attack of HF and decomposition (or oxidation) of LiPF6-based carbonate electrolytes. In this review, the structure, transport properties and different reported possible fading mechanisms of LNMO cathode are discussed detailedly. A various acceptable strategies to improve the cycling stability and rate capacity of LNMO -based batteries, such as synthesis, control of special morphologies, element doping and surface coating etc., are reviewed. According to the progress described above and our own research results, an insight into the future research and further development of LNMO cathode is discussed. Critical to the successful preparation of LNMO cathode materials is how to control the morphology, particle size and cation order. Solid-state chemistry has already established as conventional route for obtaining well-crystallized particles of LNMO with ordered P4332 and disordered Fd-3m structure with and without annealing. However, uncontrollable particle growth and agglomeration associated with solid-state synthesis usually deteriorate the electrochemical performance of LNMO. Hence, it is necessary to develop the synthesis methods with low energy consumption, cost, and processing time. Alternatively acceptable solution chemistry methods are developing with highlights on hydrothermal synthesis and coprecipitation method because of the controllable morphology, particle size and cation order. It is most probably that co-precipitation synthesis will be developed in commercially viable approach to the production of LNMO powders when considering the cost and tap density. Moreover, preparing different morphologies including coreeshell structures, spherical structures, porous structures or nano-micro structures can also improve the rate performance and cycling stability of LNMO. Our experience is that doping is an effective way to modify the electronic conductivity of LNMO materials, and the more the Ni2þ that exists in LNMO, the more discharge capacity of the doped LNMO has. Using high valence cation doping (such as Nb5þ, Mo6þ, etc.) and designing new chemical formula with high nikelcontaining doped LNMO (such as LiMn1.425Ni0.525Nb0.05O4) can improve the capacity. Developing doped high-performance LNMO using earth-abundant elements is more desirable and noble metals (such as Ru, Ag, etc.)-free, Na, Cr and Al-doped LNMO are some of the most attractive choices used in commercial LIBs. Surface coating is also an effective way to decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn or Ni dissolution during cycling test. Since most of the coating layers are not good conductors, surface coating with fast ionic conductors or by simple techniques with low-cost methods should be developed. The thickness of the coating layer must be optimized, which usually affects the electrochemical performance of LNMO. In addition, a uniform surface coating around the whole LNMO particles is also obviously difficult to achieve. Hence, the combination of element doping and surface coating may be a reasonable and wise choice for solving the dilemma between improving both specific energy density and cycling stability. It is worth noting that a single strategy sometimes does not efficiently solve the serious capacity fading of LNMO cathode, especially at elevated temperatures or high rate charge-discharge. Therefore, it can be concluded that a combination of different strategies may be efficiently ensure the cycling stability and rate capacity, such as a combination of doping and morphology control of particle, etc. In addition, systematic studies on the mechanism of lithium insertion/extraction, voltage trends of the doped cathode, ion diffusion paths and dimensionalities, intrinsic defect chemistry, and surface properties of nanostructures using first-principles

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calculations combined with experimental methods are also helpful for a better fundamental understanding of the relationship between the structure and electrochemical characteristics [345]. Considering the safety of lithium ion battery supplying the HEV, more enterprising manufacturers are considering to develop batteries with an innovative Li4Ti5O12 anode component instead of carbon. Hence, future applied research can focus on the improvement of rate performance and cycling stability for LiNi0.5Mn1.5O4/ Li4Ti5O12 full battery with a 3.2 V voltage plateau, especially at elevated temperatures. In conclusion, whichever direction the future takes, it is clear that major advances in LNMO-based batteries for EVs, HEVs or PHEVs will depend on improving high-rate performance, cycle life and rate capacity in order to make these batteries feasible for largescale commercial applications. Acknowledgments This work was financially supported by the Anhui Provincial Natural Science Foundation (no. 1508085MB25), National Natural Science Foundation of China (nos. 51274002 and 51404002), Anhui Provincial Science Fund for Excellent Young Scholars (no. gxyqZD2016066) and the Program for Innovative Research Team in Anhui University of Technology (no. TD201202). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.03.070. References €rster, W. Jaegermann, J.P. Khanderi, H. Tempel, A. Popp, [1] L. Dimesso, C. Fo J. Engstler, J.J. Schneider, A. Sarapulova, D. Mikhailova, L.A. Schmitt, S. Oswald, H. Ehrenberg, Chem. Soc. Rev. 41 (2012) 5068e5080. [2] G. Chen, J. Yang, J. Tang, X. Zhou, RSC Adv. 5 (2015) 23067e23072. [3] H. Li, H. Zhou, Chem. Commun. 48 (2012) 1201e1217. [4] J.M. Tarascon, M. Armand, Nature 414 (2001) 359e367. [5] L.J. Fu, H. Liu, C. Li, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, Solid State Sci. 8 (2006) 113e128. [6] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) 783e789. ne trier, L. Croguennec, I. Saadoune, A. Rougier, C. Pouillerie, [7] C. Delmas, M. Me s, Electrochim. Acta 45 (1999) 243e253. G. Prado, M. Grüne, L. Fourne [8] R. Yazami, Y. Ozawa, H. Gabrisch, B. Fultz, Electrochim. Acta 50 (2004) 385e390. [9] Y.K. Sun, J.M. Han, S.T. Myung, S.W. Lee, K. Amine, Electrochem. Commun. 8 (2006) 821e826. [10] J.R. Wilson, J.S. Cronin, S.A. Barnett, S.J. Harris, J. Power Sources 196 (2011) 3443e3447. [11] Z. Wang, C. Wu, L. Liu, F. Wu, L. Chen, X. Huang, J. Electrochem. Soc. 149 (2002) A466eA471. [12] P. Barpanda, S. Nishimura, A. Yamada, Adv. Energy Mater. 2 (2012) 841e859. [13] Z. Gong, Y. Yang, Energy Environ. Sci. 4 (2011) 3223e3242. [14] F. Wu, X. Zhang, T. Zhao, L. Li, M. Xie, R. Chen, J. Mater. Chem. A 3 (2015) 9528e9537. [15] J. Li, L. Wang, L. Wang, J. Luo, J. Gao, J. Li, J. Wang, X. He, G. Tian, S. Fan, J. Power Sources 244 (2013) 652. [16] J. Li, J. Wu, Y. Wang, G. Liu, C. Chen, H. Liu, Mater. Lett. 136 (2014) 282e285. [17] J. Wang, Z. Shao, H. Ru, Ceram. Int. 40 (2014) 6979e6985. [18] Y. Qiao, L. Pan, P. Jia, H. Wang, L. Kong, W. Gao, X. Wang, Mater. Lett. 137 (2014) 432e434. [19] M. Chen, Q. Ma, C. Wang, X. Sun, L. Wang, C. Zhang, J. Power Sources 263 (2014) 268e275. [20] K.S. Lee, S.T. Myung, D.W. Kim, Y.K. Sun, J. Power Sources 196 (2011) 6974e6977. [21] J. Cao, G. Hu, Z. Peng, K. Du, Y. Cao, J. Power Sources 281 (2015) 49e55. [22] Z. Quan, S. Ohguchi, M. Kawase, H. Tanimura, N. Sonoyama, J. Power Sources 244 (2013) 375e381. [23] M. Kitta, T. Akita, M. Kohyama, J. Power Sources 232 (2013) 7e11. [24] F.X. Wang, S.Y. Xiao, X.W. Gao, Y.S. Zhu, H.P. Zhang, Y.P. Wu, R. Holze, J. Power Sources 242 (2013) 560e565. [25] X. Gao, Y. Sha, Q. Lin, R. Cai, M.O. Tade, Z. Shao, J. Power Sources 275 (2015) 38e44. [26] G. Xu, Z. Liu, C. Zhang, G. Cui, L. Chen, J. Mater. Chem. A 3 (2015) 4092e4123.

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