Stabilization of a High-Capacity and High-Power Nickel-Based Cathode for Li-Ion Batteries

Please cite this article in press as: Zeng et al., Stabilization of a High-Capacity and High-Power Nickel-Based Cathode for Li-Ion Batteries, Chem (2018), https://doi.org/10.1016/j.chempr.2017.12.027

Review

Stabilization of a High-Capacity and High-Power Nickel-Based Cathode for Li-Ion Batteries Xiaoqiao Zeng,1 Chun Zhan,1 Jun Lu,1,* and Khalil Amine1,2,*

High-capacity and high-power nickel-based cathode materials have become the principal candidates for a lithium-ion energy storage system powering electrified transportation units. With high nickel content, the cathodes are of great interest for delivering the desired specific energy and energy density. However, the cells still suffer from fast capacity decay and low thermal-abuse tolerance to high voltage. At the highly delithiated state, the damage in the cell is mainly from severe parasitic reactions, including the oxygen evolution reaction in the cathode and oxidization of the organic electrolyte. These side reactions rapidly weaken the system’s rate capacity and cyclability. Solutions are being sought to provide safe operation and practical application. Three strategies have proven to be encouraging choices: surface coating, a core-shell structure, and a concentration gradient structure. For each strategy, the material architecture, fabrication procedure, operation principle, advances, and challenges are discussed in this review. The prospects for further developments are also summarized.

INTRODUCTION As a principal member of the rechargeable energy storage device family, the lithiumion battery is one of the key factors driving the global economic trend in the electrical industry affecting the automotive market.1–3 It is widely expected that partial or full replacement of the conventional internal combustion engine by environmentally friendly electric motors may be just around the corner.3–5 Accordingly, hybrid or pure electric vehicles (EVs) require a battery system with the necessary energy, power, stability, and safety to conquer the market.6,7 For vehicle applications requiring hundreds or thousands of times more power than small electronics and portable tools (i.e., hybrid, plug-in, and full EVs or even aviation units), a lithium-ion rechargeable system with the lightest metal/ion but the largest energy density for weight seems the best available option.8,9 However, when the end users are attracted by non-emission, low pollution, green technique, less maintenance, and even fashion, lithium-ion chemistry for vehicles is still a compromise. That makes this chemistry a ‘‘promising choice’’ instead of an ‘‘ideal contender,’’ especially with respect to high energy density and power.10–12 The trade-off for higher capacity is sacrificing active material loading and thermal stability, as well as boosting the cost and risk by optimization for handling high currents.12–16 Therefore, the whole lithium-ion system is still under development by focusing on the fundamental enhancement of material capacity followed by stabilization of the electrodes and electrode-electrolyte interfaces.15,17–19

The Bigger Picture In order to meet the increasing demand for electric vehicles, a powerful lithium-ion battery is required to deliver high capacity and energy density. Among all the components in a lithium-ion cell, nickel-rich cathodes have been widely developed as a dominant element determining the overall cell performance. The capacity delivered by the cathode is highly dependent on the nickel content. Although such cathodes indeed guarantee high capacity and energy density, thermal stability and cyclability are still poor because of structural instability and parasitic reactions. The poor cycle life cannot meet the requirement for thousands of cycles or 15 years of calendar life. With the aim of achieving high capacity with satisfactory battery lifetime, stabilization of the nickelbased cathode has become a globally competitive topic. The most encouraging strategies and prospects for further developments are summarized in this review.

Current industry cathode materials such as lithium cobalt oxide (LCO), lithium manganese spinel (LMO), and lithium iron phosphate (LFP) are relatively safe but deliver insufficient capacity for high power.4,18,20–22 Many attractive materials are layered

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Figure 1. Lattice Structure and Activation Barrier of Lithium Transition-Metal Oxide (A) The structure of Li(Ni 0.5 Mn 0.5 )O 2 consists of layers of transition metal (Ni and Mn) separated from Li layers by oxygen. In materials made by conventional high-temperature synthesis, partial exchange of Li and Ni ions is always observed, which contracts the space through which Li can move. (B) Li moves from one octahedral site to another by passing through an intermediate tetrahedral site where it encounters strong repulsion from a nearby transition-metal cation. The table shows the activation barrier for Li motion for various transition metals near the activated state. Values were calculated by gradient-corrected approximation density functional theory for various chemistries and Li contents. From Kang et al. 9 Reprinted with permission from AAAS.

structures with three slabs: a transition-metal slab, a lithium slab, and an oxygen slab (Figure 1A).1,9 The diffusivity of lithium depends on the energy of the active site, which is affected by the structure of tetrahedral sites and electrostatic interaction between active-state lithium and transition metal (Figure 1B).23–25 For instance, layered LCO material is widely used to power portable devices with enough energy density and long-lasting lifespan. It offers relatively low discharge current and power, but can only deliver a capacity at 150 mAh/g of the 274 mAh/g theoretical value.26 The Co3+ in LiCoO2 can offer only one electron to cycle between the cathode and anode, ending up with highly oxidized Co4+ with the highest activation barrier (see table in Figure 1). This contributes to extremely slow lithium dissociation and irreversible phase transition at Li/Co ratios lower than 0.5.1,26–28 Below this critical point, only half of the Li ions from the reservoir can be deintercalated as a result of the irreversible lattice transition from rhombohedral to monoclinic phase.29 To stabilize the rhombohedral phase for higher capacity delivery, putting the ‘‘pillar’’ nickel ions into its transition-metal layer can effectively suppress phase distortion at a lower lithium content.30–32 As well as a steady lattice structure for a stable diffusion path, the lattice provides tetrahedral sites and octahedrally coordinated lithium and nickel ions. The reduction in activation energy for Ni ions (table in Figure 1) benefits the dramatic increase in the lithium migration rate, on an exponential order.9 Preventing lattice distortion and reduced energy of the activated state and delivering high reversible capacities exceeding 150 mAh/g, the nickel-based cathode materials are an attractive alternative to traditional cathode materials.30,31,33 With fundamental enhancement of material capacity, some nickel-rich cathodes, such as LiNixCoyAlzO2 and LiNixCoyMnzO2, allow rapid solid-state lithium-ion and electron transport. Fundamentally, these lithium metal oxides were designed by substituting Co with other transition metals, maintaining the layer structure and facilitating Li+ diffusion and transportation. In LiNixCoyMnzO2, the valency of the nickel, cobalt, and manganese cation is usually divalent, trivalent, and tetravalent, respectively.9,34–36 Electrochemically inactive tetravalent manganese guarantees acceptable cell thermal behavior during operation. Trivalent cobalt assures electric conductivity, modulates cation disorder, and reduces the surface energy.37 The redox couple of Ni2+/4+ and Co3+/4+ can achieve a capacity over 200 mAh/g at 4.3 V, which is very close to the operating condition of EVs requiring both high energy and high power density.36,38,39 Nevertheless, the increased nickel content forces a

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1Chemical

Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA

2Institute

for Research and Medical Consultations, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia *Correspondence: [email protected] (J.L.), [email protected] (K.A.) https://doi.org/10.1016/j.chempr.2017.12.027

Please cite this article in press as: Zeng et al., Stabilization of a High-Capacity and High-Power Nickel-Based Cathode for Li-Ion Batteries, Chem (2018), https://doi.org/10.1016/j.chempr.2017.12.027

compromise on safety and calendar life by accelerating capacity fading during cycling at both room and high temperature. At the highly delithiated state, transformation of the structure from layered to rock-salt phase, Ni ion dissolution accompanied by electrolyte decomposition, the oxygen evolution reaction in the Ni(III) cathode, and Ni(IV) oxidization of the organic electrolyte all result in poorer thermal stability and a potential safety issue.40–43 These parasitic reactions mostly occur at the interface of the electrode and electrolyte, including Ni oxidization and electrolyte decomposition. They quickly create severe damage to the cell system, such as gas release, passivation layer formation on the cathode surface, increased impedance, suppression of lithium-ion diffusion, and even lead to battery thermal runaway.10,36,44,45 The amount of side-reaction products was observed to increase constantly, corresponding to more Ni content.46 Thus, high-power nickel-rich materials still suffer from a high safety risk and faster capacity fading, especially when the cell is cycled with a cutoff voltage exceeding 4.3 V at a temperature higher than 303 K or the overcharged state. Global efforts have been devoted to exploring effective approaches to combatting thermal abuse and poor life. A significant breakthrough has been achieved by optimizing cell chemistry to exploit the high capacity and energy density from stabilized nickel-based cathode material. This optimization is based on minimizing the direct contact between the electrolyte and the cathode to suppress the parasitic reactions.12,17,30,45 It started with a nanoscale surface coating procedure, which improves the cycle performance of the cathode. However, the coating technique limited the control of surface coverage.26,47–50 Inspired by the traditional coating procedure, an upgraded version was invented to create a well-designed core-shell structure, in which a nickel-rich material core was completely covered by a nickelpoor material shell. With enhanced lifetime and energy density, this system still suffers from phase segregation during charge and discharge.33,51–53 Eventually, a gradient core-shell structure was constructed. This unique structure is built up with the nickel concentration decreasing gradually from the core to the surface and the other transition-metal content increasing gradually from the core to the surface. The electrode stoichiometry was proved to be well maintained in such a structure. With minimal amount of nickel on the cathode surface, direct contact between nickel and electrolyte was minimized to effectively suppress oxidation of organic solvents. With maximum nickel content at or near the core position, the material is able to deliver high capacity and energy density.54–56 In the following sections, we review the research progress on advanced nickel-based cathode materials in detail, including nanoscale coating, core-shell structure, and concentration gradient structure. We then share our perspectives on the future of cathode materials meeting the requirements for EV power sources.

SURFACE COATING Since the parasitic redox reactions occur at the interface of the solid electrode and the liquid electrolyte,17,57,58 the most straightforward approach to reduce that effect is to block direct contact between these two phases by creating passive physical protection on the surface of the cathode particle or electrode. The surface coating materials are electrochemically and chemically inactive, such as metal oxides (Al2O3,26,59,60 ZrO2,61–63 SnO2,64,65 MgO,64,66–68 TiO2,69–73 SiO2,74,75 and ZnO,47,50,64), phosphates (AlPO4,76,77 and Co(PO4)3,78 Cu3(PO4)2,79 PrPO4,80 and Li3PO481), fluorides (AlF3,49,82–86 FeF3,80 CuF2,80 and LiAlF487), lithium transitionmetal oxides (Li2ZrO3,31,88 LiVO3,89 and LiAlO290), interfacial layers,43,91–94 and conductive polymers87,95–100. Some protection also functions as an artificial solid

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Figure 2. Typical Synthesis Process of Co-precipitated Surface Coating on a Pristine Cathode Particle

electrolyte interface (SEI) layer43,91–94 to mitigate the side reactions. These artificial SEI layers usually consist of organic species91 such as alkyl lithium carbonate (LiCO3R) and lithium carbonate (Li2CO3). This protection usually shows thermal stability in the electrolyte and the metal ions are inert for the redox reaction within the working voltage range of the cell. Validated by high-precision leakage current measurements, the suppression of parasitic reactions at the electrode-electrolyte interface has been demonstrated by a low amount of collected leakage current.17,101 To fully realize the cell’s working functions, these coatings should be firm enough to withstand exothermic side reactions for long cycling and to provide an ultra-thin transverse dimension that allows fast Li+ transportation, sufficient electron conductivity, and good electrode mechanical stability. In most circumstances, the inorganic surface coatings are fabricated by a wet-chemical process or deposition technique (Figure 2).59,69,70,88,95,102 Among them, AlF3 shows superior protection against acidic electrolyte decomposition products, especially HF.82,84,86 The protection from AlF3 has proved to significantly improve the stability of cathodes on the basis of nickel-rich layered LiNi0.8Co0.1Mn0.1O2, delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2, and lithium-rich layered Li[Li0.19Ni0.16Co0.08Mn0.57] O2.49,103–105 This concept was derived from the low stability of the Al2O3 coating layer when exposed to trace HF generated from LiPF6 decomposition during deep cycling. Gradually, the Al2O3 protection is consumed and transferred to AlF3 or Al-O-F products. The fragments of these products detach from the cathode surface, harming both the cycling and rate capability.48,82,105 Directly constructing an AlF3 buffer layer on the cathode surface (Figure 3A) provides good resistance to HF attack at 298–328 K within the voltage range 2.7–4.6 V. Also, the thermal stability as a result of the surface-coated cathode material at a highly delithiated state was improved as determined by differential scanning calorimetry (DSC).103 Charged to 4.6 V, AlF3coated Li[Li0.19Ni0.16Co0.08Mn0.57]O2 electrodes showed an exothermal reaction (peak) shifting to higher temperature (Figure 3B). The heat generation was remarkably reduced by the surface coating, indicating enhanced thermal stability. Moreover, oxygen evolution was associated with irreversible phase transition from a rhombohedral layer to a cubic spinel structure and was proved to be suppressed by the thin coating layer from thermogravimetric analysis. Accordingly, the thinner the cathode-electrolyte interface, the weaker the oxidation of solvent. The approach is helpful for capacity retention and also significantly lowers charge-transfer impedance during cycling in terms of enhanced rate capability. For the same purpose, organic conductive polymers are very attractive because these are conjugated with extra electrons in the double bonds responsible for better conductivity.95,100 They have been adopted as binder and electrode surface

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Figure 3. Functions of AlF3 Surface Coating (A) TEM image of primary particles from a 5 wt % AlF 3 -coated sample that was completely encapsulated by the AlF 3 coating. (B) Differential scanning calorimetry (DSC) trace of pristine and AlF 3 -coated electrodes charged to 4.6 V. Adapted with permission from Sun et al. 103 Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

protection, benefiting both adhesive and conductivity.96–98,106 Typical conductive polymer protection includes thin layers of polyacetylene, polypyrrole, poly(phenylenevinylene), etc.96,98 Some can even contribute a small amount of capacity during cycling.98 However, the soft nature of those polymers ultimately affects the porosity, resulting in lower ionic conductivity. Therefore, introducing an inorganic conductive agent into the conductive polymer structure currently has become a more suitable approach to ideally enhance the electrode performance.96,99 The surface protection technique has the advantage of relatively easy manipulation and low cost, presenting great potential for industrialization. However, the methodology is limited to the particle surface and does not enhance the quality of the individual particle. As a typical post treatment, this approach does not strengthen any intrinsic property of the original particle, which plays a dominating role in the electrochemical performance of the cell. Correspondingly, the opportunity for such enhancement is ultimately limited by the nature of the pristine material. Compared with the deposition technique, wet chemistry synthesis is more attractive for scale-up production. Yet, it still very challenging to control the coverage and uniformity of the coating when extending the capacity 1,000-fold. At this point, the active material particles are likely covered by discrete domains with different thickness. Although the uncovered surface is essential to maintain lithium diffusion and the charge exchange rate, exposure to the electrolyte easily triggers parasitic reactions leading to instability. In addition, the disconnected coating layers suffer weak adhesion to the particle surface, and usually detach from the hosts after long cycling time. Optimizing the application of the surface technique requires balancing structure uniformity, coating thickness, and material conductivity with strong surface bonding and is of vital importance for both lab fabrication and industrial production lines.

CORE-SHELL STRUCTURE A core-shell structure is a good way to implement uniform encapsulation on Ni-rich cathode materials. With a similar crystallographic structure as the high capacity core, the shell component is a thermally stable lithium metal oxide (e.g., Li[Ni0.5Mn0.5]O2) exhibiting a high exothermic decomposition temperature.53 This delicate design is desirable to ensure the adhesion and conductivity of the shell and to prevent the phase segregation or separation that happens during chemical synthesis and

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Figure 4. Typical Synthesis Process of a Microscale Spherical Core-Shell Particle

electrochemical cycling. The electrochemically active shell is required to maintain the charge transport path from the core material to the electrolyte.107,108 A typical micro-scale spherical core-shell particle was prepared via the co-precipitation procedure shown in Figure 4. The synthesis usually starts with a Ni-rich hydroxide core (Li[Ni0.8Co0.1Mn0.1]O2, Li[Ni0.8Co0.2]O2) aiming at high capacity. After stepby-step [Ni0.5Mn0.5](HO)2 precursor co-precipitation and high-temperature lithium incorporation, a coating over a homogeneous thermally stable Li[Ni0.5Mn0.5]O2 shell secures sustainable capacity and reliable safety.53,109 The structure of the particle obtained1,11 has the core completely encapsulated by the shell (Figure 5). It has a clear core-shell boundary with continuous crystalline material from the particle center to the surface. The thickness of the Li[Ni0.5Mn0.5]O2 shell is controllable by adjusting the precipitant, including speed and amount, to properly balance the thermal stability and capacity delivery. Optimized calcination conditions prevent ionic diffusion between the core and shell and make it possible to retain the original coreshell morphology of the precursor (Figure 6).109,110 Despite the distinction between the morphology of the core and shell, only one set of diffraction patterns of a typical layered phase with R-3m symmetry can be observed by X-ray diffraction screening. As the shell thickness increases, the relative diffraction intensity decreases.109 The result is a similar lattice structure of the core and shell crystals without phase segregation or separation.51,53,108 According to electrochemical testing and the DSC profiles, with a thicker shell layer, the cathode showed a lower initial cycle irreversible capacity, longer cycle life, and better thermal stability.53,108–110 The superior capacity retention during long cycling is attributable to the effective shell protection: fully covering the core particle, blocking the HF attack on the core, and suppressing the Co dissolution from the core. On the other hand, the capacity of the shell (e.g., 150 mAh/g53,109,110 for Li[Ni0.5Mn0.5] O2) is usually lower than that of the core (e.g., 200 mAh/g for Li[Ni0.8Co0.1Mn0.1]O2). Thus, a thicker shell always increases the charge potential slightly because of its intrinsic property. Consequently, a small capacity loss of the core-shell cathode always occurs when the cell is operated at the commonly used voltage range of 3.0–4.3 V. Different from the surface coating, the reason for the capacity decay of the core-shell structure during long-time cycling at a high rate is the incompatible volume change of the core (9%–10%) and shell (2%–3%).111 During rapid charge and discharge, unsynchronized volume changes of the core and shell parts occur at different levels. Therefore, many nanoscaled cavities or gaps are generated at the interface as a result of the volume difference between the core and shell and eventually result in phase separation between the two parts. Those cavities cut off the ionic transport pathway from the core to the shell and lead to capacity fade and a decay in rate performance.55,112 The core-shell structure paves a new way

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Figure 5. SEM Cross-sectional Images of the Core-Shell Structure [Ni0.8 Co 0.1 Mn 0.1 ](OH) 2 encapsulated by [Ni 0.5 Mn 0.5 ](OH) 2 for (A) 20 min, (C) 40 min, (D) 2 hr, (E) 3 hr, and (F) 4 hr; (G) core Li[Ni0.8 Co 0.1 Mn 0.1 ]O2 ; (H) thermally lithiated product of (B); (I) lithiated product of (C); (J) lithiated product of (D); (K) lithiated product of (E); and (L) lithiated product of (F). The white arrows indicate the interfaces of core and shell regions. Scale bars represent 4 mm. Adapted with permission from Sun et al. 109 Copyright 2006 American Chemical Society.

for the production of reliable and safe cathode materials with high energy density and long cycle life for hybrid and all-electric vehicles. Nevertheless, more detailed optimization is required to obtain stable materials with continuous structural deviation from the surface to the core of the cathode particles.

CONCENTRATION GRADIENT STRUCTURE Encapsulating the Ni-rich core (e.g., Li[Ni0.8Co0.1Mn0.1]O2, Li[Ni0.8Co0.2]O2, etc.) with transition-metal elements in a concentration gradient shell was investigated to prevent structural mismatch of the traditional core-shell families.33,51,111 The gradient structure was prepared by a co-precipitation reaction taking place in a continuously stirred tank reactor equipped with a pH indicator and thermal controller. The Ni, Co, and Mn precipitant sources forming the shell were gradually pumped into the reactor with tuned concentration. Each particle obtained from this method consisted of a Ni-rich high-capacity bulk inner core surrounded by a concentration gradient outer shell.55,56 From the inner to the outer zone of the shell, the Ni ions were gradually replaced by Mn ions in order to achieve high capacity with outstanding cycle life and safety. From electron-probe X-ray micro-analysis (EPMA), the composition in the shell varied from Li[Ni0.8Co0.1Mn0.1]O2 to Li[Ni0.46 Co0.23Mn0.31]O2, and a calculated nominal stoichiometry of Li[Ni0.64Co0.18Mn0.18] O2 was confirmed by atomic absorption spectroscopy.111 Especially, the chemical composition at the inner shell was similar to the bulk core at both the precursor stage and the final stage after heat treatment, preventing structural mismatch during long cycling. Such structural stability enhancement was evaluated by comparing the highresolution transmission electron microscopy (TEM) images of the particles with and

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Figure 6. SEM and Electron-Probe X-Ray Microanalysis of the Core-Shell Structure SEM photograph (A) and EPMA line scan (B) of precursor hydroxide and SEM photograph (C) and EPMA line scan (D) of the final lithiated oxide Li[Ni0.64 Co 0.18 Mn 0.18 ]O2 . In both cases, the gradual concentration changes of Ni, Mn, and Co in the interlayer are clearly evident. The Ni concentration decreases and the Co and Mn concentrations increase toward the surface. Adapted by permission from Springer Customer Service Centre GmbH: Springer Nature, Nature Materials Sun et al., 111 copyright 2009.

without a concentration gradient shell (CGS).55 Without the CGS, a typical electron diffraction pattern of the (001) zone of the spinel structure was detected for the Li [Ni0.8Co0.2]O2 part after 50 cycles. This finding unequivocally confirms that the layered-structure phase transferred to spinel phase during cycling. Bright-field TEM revealed a thin amorphous layer on the cycled Li[Ni0.8Co0.2]O2 particle, which likely developed from HF attack of the particle surface. The (003) lattice fringes were mostly discontinuous, associated with dislocation at several areas. Unlike the highly disordered Li[Ni0.8Co0.2]O2, the CGS Li[Ni0.72Co0.18Mn0.1]O2 particle showed no conclusive evidence for structural degradation after cycling. The TEM result clearly attests to the superiority of the CGS structure in maintaining the structural integrity of the electrode during lithium intercalation. The R-3m structure remains stable in the cycled CGS particle. The particle surface is also free of any secondary phase. The stable shell part and its high compatibility with the core part protect the electrode surface from HF attack at the particle level. Also, it impedes the detrimental structural transition arising from periodic lithium intercalation-deintercalation. Compared with the shells of the traditional core-shell structure, the protection ability of the CGS is definitely stronger. However, the structure always ends up with

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Figure 7. Typical Synthesis Process of a Full Concentration Gradient Particle

relatively high Ni content on the surface and relatively low Mn content.51,113,114 For long-time fast rate cycling at the elevated temperature and high voltage specially required by a hybrid or all-electric vehicle, the cathode particle still needs advanced optimization. The strategy is to seek a structure with less Ni and more Mn at the surface zone compared with CGS, but possessing a high-capacity Ni-rich content condensed within the center of the particle. According to this strategy, the dimension of the concentration gradient was extended from the near surface to the deep center of the particle, called a full concentration gradient (FCG).54,55,60,107 The FCG was prepared by gradually decreasing the Ni concentration and increasing the Mn concentration from the inner to the outer part (Figure 7) of an individual particle. With the high inner Ni content at around 0.86, the FCG material is able to achieve a first cycle capacity exceeding 215 mAh/g. Meanwhile, the Mn content increases from 0.04 in the center to 0.2 on the surface, and the oxidation of electrolyte by the highly active Ni ions is effectively suppressed. The cathode can retain a capacity of 190 mAh/g after 100 cycles running in the voltage window of 2.7–4.5 V.107,115 This improvement is obviously dramatic in comparison with the performance of the cathode material with the same inner high nickel composition: the capacity loss after 100 cycles is usually about 50% of its initial value as a result of the aggressively accelerated irreversible reactions on the cathode-electrolyte interphase at such a high cutoff voltage. Another superior property of the FCG structure over the CGS core shell, traditional core shell, and surface coating structure is the radial continuity of the composition as well as the crystal lattice structure. Obviously, the scanning electron microscopy (SEM) cross section of the particle shows homogeneous morphology without any inter phase (Figures 8A and 8B).107 The Ni and Mn concentration deceases or increases roughly according to a linear relationship with the scale of dimensions from the center to the edge according to quantity analysis by the EPMA line scan (Figures 8C and 8D). Hard X-ray nanotomography (Figures 9 A and 9B) of the Ni concentration shows needle-shaped spikes pointing from the particle center toward the edge. This structure was clearly captured by TEM (Figures 9C and 9D) as highly aligned large-aspect-ratio nanorods.107 Such a radial nanopattern is related to the diffusion order of the transition-metal cations during the calcination process as a result of the concentration gradient in the hydroxide precursor. The highly percolated nanorod network is energetically preferred for the diffusion of the transition-metal ions from the center to the surface of the particle. The nanorod network also provides a short pathway for Li-ion diffusion from the surface to the bulk.55,107,115 By maximizing the average Ni concentration at the center core as active redox species, as well as the Mn concentration near the particle outer surface, the FCG concept was extended to establish a new system with a two-sloped full concentration gradient (TSFCG) of Ni, Co, and Mn ions throughout a

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Figure 8. Schematic Diagram of the FCG Lithium Transition-Metal Oxide Particle with a Decreasing Nickel Concentration from the Center toward the Outer Layer and a Corresponding Increasing Concentration of Manganese SEM mapping photograph of Ni, Co, and Mn within a single particle for the precursor (A) and for the lithiated material (B). EPMA line scan of the integrated atomic ratio of transition metals is shown as a function of the distance from the particle center to the surface for the precursor (C) and the lithiated material (D). The Ni-rich particle center and Mn-rich outer surface are clearly seen from the SEM mapping images. The Ni concentration decreased linearly toward the particle surface for both the precursor and the lithiated particle, whereas the Mn concentration increased, and the Co concentration remained constant. Adapted by permission from Springer Customer Service Centre GmbH: Springer Nature, Nature Materials Sun et al., 107 copyright 2012.

Li[Ni0.65Co0.13Mn0.22]O2 cathode particle.116,117 The Ni-enriched core boosted even higher capacity and the Mn-enriched surface strengthened the safety protection with well-maintained rod-shaped primary particles during deep cycling.33,97,116 For the two slopes: (1) the CGS was smooth for each transition metal throughout the particle core and (2) abruptly changed near the particle surface. To realize this concept, the injection concentration of Ni, Co, and Mn source was accurately controlled within micrometer range in the formation of the precursor. The synthesis processes are promising at higher Ni average concentration, allowing control of the stoichiometry as desired from Li[Ni0.89Co0.05Mn0.06]O2 to Li[Ni0.79Co0.06Mn0.15] O2.117 With such a delicate design, for instance, this new type of cathode with highly Ni-enriched inner bulk and significant Ni-reduced surface (center Li[Ni0.72 Co0.11Mn0.17]O2 to surface Li[Ni0.60Co0.12Mn0.28]O2) was capable of delivering a capacity more than 200 mAhg 1 at 4.3 V with extraordinary cycle life and thermal stability. As a result, being optimized from the primary particle level, the FCG and TSFCG cathode offered a higher Li-ion diffusion coefficient and, therefore, better rate performance than the cathodes formed by either the pure inner or outer compositions. A full cell constructed with a CNT-Si counter electrode delivered an energy density of 350 Why kg 1 with stable cyclicality for 500 cycles,117 making this concept glamorous for next-generation EV applications.

CONCLUSIONS AND PERSPECTIVES The Ni-based cathode family has a proven track record for developing high-energy Li-ion batteries to power EVs. These materials are able to achieve improved rate capability, cycle performance, and safety characteristics with advanced designs such as a concentration gradient structure. This structure with a Ni and Mn concentration gradient from the center to the edge of the cathode particle takes advantage of the high capacity of the Ni-rich material and the high thermal stability of the Mnrich material. Not only can the concentration gradient structure be applied to a

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Figure 9. Hard X-Ray Nanotomography and TEM Images (A) X-ray nanocomputed tomography image of the 3D distribution of nickel concentration in a single lithiated FCG lithium transition-metal oxide particle. (B) 2D distribution of nickel on a plane going through the center of the particle. The high nickel content regions shown as bright areas tend to form needle-shaped spikes radiating outwards from a 2 mm central core. (C) TEM image of the local structural feature near the edge of the particle shows highly aligned nanorods. (D) TEM image of the local structural feature at the center of the particle shows that an aligned nanorod network at the particle center was not developed. Adapted by permission from Springer Customer Service Centre GmbH: Springer Nature, Nature Materials Sun et al., 107 copyright 2012.

broad range of functional cathodes, the gradient idea also has the potential to be extended: for example, to the distribution gradient of LiM6 units in the transitionmetal layer in the Li- and Mn-rich layered-layered material [Li2MnO3$xLiMO2 (M = Co, Mn, Ni, etc.)], or the phase gradient from the layered structure in the center to the spinel structure on the surface. Although there have been great efforts to address the cathode side, we cannot ignore the fact that lithium battery chemistry is an integrated system with multiple components. Besides dealing with the cathode issue, extensive research and development on the electrolyte and anode have been carried out, such as novel additives for the electrolyte and new materials and structures for the anode. The strategies are always more welcome when they profit the whole cell than simply focusing on a single component. Optimizing the electrolyte is one of those strategies because it is the bridge connecting the two electrodes. To date, commercialized electrolytes are stable at the cathode-liquid interface for portable electronic consumers, but not sufficient for high-voltage chemistry

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especially at the cutoff voltage over 4.5 V.118,119 The high-power regulations require electrolyte designs for new systems, solvents, and especially additives, for better rate capacity, cyclability, and coulombic efficiency. Borates and boroxines and their hybridized salts (lithium bis(oxalato)borate, lithium difluoro oxalato-borate, trimethyl boroxine, etc.), for example, were developed to benefit both a Ni-rich cathode and a graphitic anode with constantly reduced full cell impedance.120–122 In these cases, the bis(oxalato)borate or difluoro oxalato-borate anion was designed to be primarily oxidized at the electrolyte-cathode interface, suppressing the oxidation of the electrolyte solvent as well as the dissolution of Mn(II).123 Consequently, they enhanced the cycling electrochemical performance toward higher operation voltages. The fluorinated124–126 or phosphorus-based127,128 additives (lithium 2-trifluoromethyl-4,5-dicyanoimidazole, tris (hexafluoroisopropyl)phosphate, etc.) were also applied to minimize the impedance on the cathode side because of its lower oxidation state, hence extending the full cell lifetime. Other redox shuttle additives129 with suitable oxidation potential are promising designs for high-nickel cathode cells with the superior function of overcharge protection at elevated temperatures. Solid-state electrolyte provides better opportunities for battery safety because the gel polymer electrolyte makes it more practical with less polymer loading and more Li+ transportation. With the assistance of the thin-film coating technique,130–132 the homogeneously deposited solid electrolyte layer may further improve ion conductivity, which was restrained by metal oxide protection, and open up a new avenue for better performance with higher safety. Ultimately, how to optimize each component to virtually ameliorate the entire cell working system is still a broad and challenging topic. The innovative and rational design of Ni-based systems will be one of the keys for practical battery applications meeting the energy, power, and safety demands of the vehicle industry.

ACKNOWLEDGMENTS This work is supported by the US Department of Energy (DOE) under contract DE-AC0-206CH11357, and the main support was provided by the Vehicle Technologies Office of the DOE Office of Energy Efficiency and Renewable Energy. The Argonne National Laboratory is supported by the Basic Energy Sciences program of the US DOE Office of Science under contract no. DE-AC02-06CH11357.

AUTHOR CONTRIBUTIONS X.Z., C.Z., J.L., and K.A. wrote the manuscript. J.L. and K.A. supervised the content of the article. All authors discussed and commented on the manuscript.

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