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Electrochemical performance of all-solid-state lithium secondary batteries with Li–Ni–Co–Mn oxide positive electrodes
Electrochimica Acta 55 (2010) 8821–8828
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Electrochemical performance of all-solid-state lithium secondary batteries with Li–Ni–Co–Mn oxide positive electrodes Hirokazu Kitaura, Akitoshi Hayashi, Kiyoharu Tadanaga, Masahiro Tatsumisago ∗ Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e
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Article history: Received 25 December 2009 Received in revised form 21 July 2010 Accepted 24 July 2010 Available online 13 August 2010 Keywords: All-solid-state battery Electrode–electrolyte interface Rate performance Lithium nickel cobalt manganese oxide Li2 S–P2 S5 solid electrolyte
a b s t r a c t LiNi1/3 Co1/3 Mn1/3 O2 was applied as a promising material to the all-solid-state lithium cells using the 80Li2 S·19P2 S5 ·1P2 O5 (mol%) solid electrolyte. The cell showed the first discharge capacity of 115 mAh g−1 at the current density of 0.064 mA cm−2 and retained the reversible capacity of 110 mAh g−1 after 10 cycles. The interfacial resistance was observed in the impedance spectrum of the all-solid-state cell charged to 4.4 V (vs. Li) and the transition metal elements were detected on the solid electrolyte in the vicinity of LiNi1/3 Co1/3 Mn1/3 O2 by the TEM observations with EDX analyses. The electrochemical performance was improved by the coating of LiNi1/3 Co1/3 Mn1/3 O2 particles with Li4 Ti5 O12 film. The interfacial resistance was decreased and the discharge capacity was increased from 63 to 83 mAh g−1 at 1.3 mA cm−2 by the coating. The electrochemical performance of LiNi1/3 Co1/3 Mn1/3 O2 was compared with that of LiCoO2 , LiMn2 O4 and LiNiO2 in the all-solid-state cells. The rate capability of LiNi1/3 Co1/3 Mn1/3 O2 was lower than that of LiCoO2 . However, the reversible capacity of LiNi1/3 Co1/3 Mn1/3 O2 at 0.064 mA cm−2 was larger than that of LiCoO2 , LiMn2 O4 and LiNiO2 . © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Recently, environmental and energetic concerns have been growing and electric vehicles and hybrid electric vehicles have been developed. Lithium-ion batteries are attracting much attention as power sources for eco-cars owing to their great merits compared with other batteries. However, safety issues of lithium-ion batteries have to be solved to build large-scale battery packs. All-solid-state lithium secondary batteries using inorganic solid electrolytes have attracted attention because they have an advantage in safety [1–4]. To construct all-solid-state lithium batteries, the solid electrolytes with high lithium ion conductivities are required. Glass–ceramic electrolytes in the system Li2 S–P2 S5 have high lithium ion conductivities of about 10−3 S cm−1 and were used as solid electrolytes in the all-solid-state lithium batteries [5–8]. Researching electrode materials suitable for all-solid-state batteries is an important work to achieve the performance needed for eco-cars. Layered lithium cobalt oxide (LiCoO2 ) is the most common positive electrode material in lithium ion secondary batteries [9–11]. LiCoO2 has been used in commercially available lithium ion batteries over the past decade because LiCoO2 shows excellent electrochemical performance. However, its high cost and relatively small capacity (140–150 mAh g−1 ) make it difficult to be used in
∗ Corresponding author. Tel.: +81 72 2549331; fax: +81 72 2549331. E-mail address: [email protected] (M. Tatsumisago). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.07.066
large-scale batteries, and many researchers have explored better active materials. Spinel lithium manganese oxide (LiMn2 O4 ), layered lithium nickel oxide (LiNiO2 ) and their derivatives have been often studied as strong candidates for the 4 V class positive electrode materials of lithium-ion batteries [12–16]. The manganese or nickel compounds with abundance and environmental friendliness are suitable materials for the increase of production of large-scale batteries. The cells using LiMn2 O4 show higher charge–discharge voltage of around 4.0 V (vs. Li) and higher safety even at overcharge condition compared with the cell using LiCoO2 . However, the practical capacity of LiMn2 O4 (100–140 mAh g−1 ) is slightly smaller than that of LiCoO2 and the cell using LiMn2 O4 show capacity degradation during cycling at 60 ◦ C. LiNiO2 is isostructural with layered structure of LiCoO2 and was found to give high capacity (180–200 mAh g−1 ). However, there are several drawbacks such as difficulty to prepare stoichiometric compound, low thermal stability and poor capacity retention during long cycling. These 4 V class positive electrode materials were used in the all-solid-state lithium secondary batteries with sulfide-based solid electrolytes [17–19]. We applied LiCoO2 and LiMn2 O4 to the allsolid-state cells with Li2 S–P2 S5 solid electrolytes and investigated their charge–discharge performance and interfacial properties [18,20,21]. The all-solid-state cells using LiCoO2 exhibited a long cycle performance with the reversible capacity of over 100 mAh g−1 under a current density of 0.064 mA cm−2 [22]. Although the LiMn2 O4 showed relatively small capacity of 55 mAh g−1 in the allsolid-state cells, a long cycle life was achieved. These cells showed
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Fig. 1. FE-SEM images of (a) LiNi1/3 Co1/3 Mn1/3 O2 , (b) LiCoO2 , (c)LiMn2 O4 , (d) LiNiO2 particles and (e) Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 .
similar impedance behaviors after initial charge process; the resistance attributable to the interface between positive electrode and solid electrolyte was observed [18,20]. The electrode–electrolyte interface was also analyzed by the TEM–EDX measurements and the diffusion of the transition metal elements from active materials into the solid electrolyte was detected in the vicinity of active material particles [18,21]. Ohta et al. demonstrated that the interfacial resistance between LiCoO2 and the Li3.25 Ge0.25 P0.75 S4 electrolyte was decreased by oxide coatings [23]. Surface coatings of active material particles were also effective for decreasing the interfacial resistance in the all-solid-state cells using the Li2 S–P2 S5 electrolytes [20]. As a result, the rate performance of the all-solid-state cells was improved. Lithium nickel–cobalt–manganese oxide (LiNi1/3 Co1/3 Mn1/3 O2 ) was first proposed by Ohzuku et al. and larger capacity (160 mAh g−1 in the voltage range 2.5–4.4 V and 200 mAh g−1 in the voltage range 2.5–4.6 V) and higher thermal stability were reported [24–26]. From the viewpoints of both economy and environment conservation, less cobalt content of LiNi1/3 Co1/3 Mn1/3 O2 compared with LiCoO2 is one of the advantages of this material. In the present study, all-solid-state cells using LiNi1/3 Co1/3 Mn1/3 O2 active materials were assembled and their electrochemical properties were investigated. The 80Li2 S·19P2 S5 ·1P2 O5 (mol%) glass–ceramic with a high lithium ion conductivity was used as a solid electrolyte [27]. The interfacial analyses between the LiNi1/3 Co1/3 Mn1/3 O2 electrode and the Li2 S–P2 S5 electrolyte were carried out by the impedance mea-
surements and transmission electron microscopy. The surface of LiNi1/3 Co1/3 Mn1/3 O2 particles was coated with the Li4 Ti5 O12 film to improve the rate performance. Lithium nickel oxide (LiNiO2 ) was also applied to the all-solid-state cells. The electrochemical performance of the cells using LiNi1/3 Co1/3 Mn1/3 O2 was compared with those using LiCoO2 , LiMn2 O4 and LiNiO2 positive electrodes. 2. Experimental Lithium nickel–cobalt–manganese oxide particles (LiNi1/3 Co1/3 Mn1/3 O2 , Nippon Chemical Industrial Co.) were used as an active material for all-solid-state cells. It was confirmed by using X-ray diffraction (XRD-6000; Shimadzu Co.) and fieldemission scanning electron microscope (FE-SEM/S4500; Hitachi, Ltd.) measurements that the particles had layered ␣-NaFeO2 structure and that their diameter was about 10 m as shown in Fig. 1(a). LiCoO2 and LiMn2 O4 particles were supplied from Toda Kogyo Co. and their morphology was shown in Fig. 1(b) and (c), respectively. LiNiO2 was synthesized by the solution method [28]. Lithium nitrate (LiNO3 , Wako Pure Chemical Industries, Ltd.), nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, Wako Pure Chemical Industries, Ltd.) and citric acid (C2 H5 O7 , Wako Pure Chemical Industries, Ltd.) were used as starting materials. LiNO3 and Ni(NO3 )2 ·6H2 O were dissolved in distilled water at 70–80 ◦ C. The aqueous solution was mixed with citric acid aqueous solution and was stirred for 24 h. After drying at 80 ◦ C, calcination processes at 600 ◦ C for 5 h in air
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and 800 ◦ C for 10 h in O2 atmosphere were performed. The obtained powder was identified as LiNiO2 by the XRD measurement and the particles had a large size distribution from 1 to 10 m as shown in Fig. 1(d). The amorphous lithium titanate (Li4 Ti5 O12 ) was coated on the LiNi1/3 Co1/3 Mn1/3 O2 particles by the sol–gel method [23,29]. Li4 Ti5 O12 sol was prepared from lithium ethoxide and titanium tetraisopropoxide. The LiNi1/3 Co1/3 Mn1/3 O2 particles were put into the sol and the sol was dried with stirring. The LiNi1/3 Co1/3 Mn1/3 O2 particles with the Li4 Ti5 O12 gel film were heated at 350 ◦ C for 30 min. Finally, we obtained the LiNi1/3 Co1/3 Mn1/3 O2 particles coated with Li4 Ti5 O12 film. Fig. 1(e) shows the SEM image of the Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 particles. The morphology such as size and shape of the particles was similar to that of the noncoated particles as shown in Fig. 1(a). The thickness of coating layer with less than 1 nm was estimated from the BET surface area of LiNi1/3 Co1/3 Mn1/3 O2 particles and weight of Li4 Ti5 O12 sol. The electronic conductivities of active materials were measured by applying a constant d.c. voltage to the pellets. A pair of stainless-steel plates as non-blocking electrodes was attached to both faces of the pellet. The electronic conductivities were calculated from the current values and applied voltage by using a Potentiostat/Galvanostat device (Solartron 1287 coupled with Solartron 1260). All-solid-state electrochemical cells were assembled as follows. The 80Li2 S·19P2 S5 ·1P2 O5 (mol%) glass was prepared by the mechanochemical reaction process [27]. The glass–ceramic electrolyte was prepared by crystallization of the glass. The obtained glass–ceramic was used as a solid electrolyte for all-solid-state cells. A composite electrode was prepared as a working electrode by mixing an active material (38.5 wt.%), the solid electrolyte (57.7 wt.%), and vapor grown carbon fiber (3.8 wt.%) powders. The composite electrode (10 mg) and the solid electrolyte powder (80 mg) were set in a polycarbonate tube (˚ = 10 mm) and were then pressed under 360 MPa. Then In foil (thickness, 100 m) was put on the solid electrolyte layer as a counter and reference electrode; the pressure of 240 MPa was applied to the three-layered pellet and then relieved. The estimated thickness of the solid electrolyte layer and the composite electrode layer were about 600 m and less than 100 m. Finally, the pellet sandwiched by two stainless-steel rods as current collectors was obtained. All processes were performed in an Ar-filled glove box. Electrochemical tests were conducted at 25 ◦ C in an Ar atmosphere using charge–discharge measuring devices (BTS-2004; Nagano Co.). Electrochemical impedance measurements were performed for the electrochemical cells, which were charged to the different depths at the constant current conditions and then aged for several hours. An impedance analyzer (Solartron 1287 coupled with Solartron 1260) was used and a small perturbation voltage of 50 mV in the frequency range of 1–10 MHz was applied for the measurements. The interface between an active material particle and the solid electrolyte in the composite electrode layer was analyzed using a transmission electron microscope (TEM, JEM2100, JEOL). Focused ion beam (FIB) milling was done for sample-preparation for cross-sectional TEM observation. Elemental point analysis for the cross-section of the composite electrode layer was carried out using energy dispersive X-ray spectroscopy (EDX, JED-2300T, JEOL).
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Fig. 2. Charge–discharge curves of the In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/ cell (solid line) and the In/80Li2 S·19P2 S5 ·1P2 O5 LiNi1/3 Co1/3 Mn1/3 O2 glass–ceramic/Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 cell (dashed line) at a constant current density of 0.064 mA cm−2 in the voltage range between 2.5 and 4.4 V (vs. Li).
versus the Li–In electrode as a counter electrode; the vertical axis on the right side denotes the potential versus Li electrode, which was calculated based on the potential difference between Li–In and Li [30]. The cell was charged and discharged between 2.5 and 4.4 V (vs. Li) at a current density of 0.064 mA cm−2 . The cell showed the typical charge–discharge curves of LiNi1/3 Co1/3 Mn1/3 O2 . The initial charge capacity of about 160 mAh g−1 was obtained and the cell showed the reversible capacity of about 115 mAh g−1 . Fig. 3 shows the impedance spectrum of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cell charged up to 4.4 V (vs. Li) at 0.064 mA cm−2 . The cell showed the three resistance components in the high (R1 ), middle (R2 ) and low (R3 ) frequency regions. The impedance spectrum was resemble to that observed in the In/LiCoO2 and In/LiMn2 O4 cells using Li2 S–P2 S5 solid electrolyte after initial charging [18,20]. We analyzed the impedance spectra in the cells using LiCoO2 and LiMn2 O4 in the previous study and we thus presumed that the resistance of R1 , R2 and R3 in the allsolid-state cell using LiNi1/3 Co1/3 Mn1/3 O2 were ascribed to the solid electrolyte layer, LiNi1/3 Co1/3 Mn1/3 O2 electrode-solid electrolyte interface and counter electrode, respectively. In order to confirm that R2 was attributed to the electrode–electrolyte interface, Li4 Ti5 O12 coating layer was introduced into the interface between LiNi1/3 Co1/3 Mn1/3 O2 and solid electrolyte. Fig. 4 shows the impedance spectra of the all-solid-state cells using (a) noncoated and (b) Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 after the first charge process to 4.4 V (vs. Li). The difference of R1 between both the cells is ascribed to the
3. Results and discussion Fig. 2 depicts the charge–discharge curves (solid line) of the all-solid-state In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/noncoated LiNi1/3 Co1/3 Mn1/3 O2 cell. In the voltage profiles of the all-solidstate cells, the vertical axis on the left side denotes the potential
Fig. 3. Impedance spectrum of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cell after charging up to 4.4 V (vs. Li) at 0.064 mA cm−2 . Three resistance components in the high (>100 kHz), middle (peak top frequency of 250 Hz) and low (<10 Hz) frequency regions are denoted by R1 , R2 and R3 , respectively.
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Fig. 4. Impedance spectra of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cells using (a) noncoated-LiNi1/3 Co1/3 Mn1/3 O2 and (b) Li4 Ti5 O12 coated-LiNi1/3 Co1/3 Mn1/3 O2 charged up to 4.4 V (vs. Li) at 0.064 mA cm−2 .
use of different batches of the solid electrolyte, which exhibited somewhat different conductivity; we confirmed that the R1 resistances were consistent with the resistances estimated from the conductivity of each solid electrolyte layer in both cells. The R2 resistances of the cells using noncoated and Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 were about 200 and 100 , respectively. The interfacial resistance was decreased by the coating and then the coating effects on the electrochemical performance were investigated. The charge–discharge curves (dashed line) of the all-solid-state In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/Li4 Ti5 O12 coated LiNi1/3 Co1/3 Mn1/3 O2 cell were also depicted in Fig. 2. The reversible capacity increased by the coating. The cycle performance of the all-solid-state cells using noncoated and Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 at the current density of 0.064 mA cm−2 was shown in Fig. 5. Both the cells retained the reversible capacity after second cycle and the all-solid-state cell using Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 retained the capacity of about 120 mAh g−1 after 10 cycles. Fig. 6 shows the discharge curves of the all-solid-state cells using noncoated and Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 in dependence of current density. The cells were charged to 4.4 V (vs. Li) at a constant
Fig. 5. Cycle performance of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cells using noncoated-LiNi1/3 Co1/3 Mn1/3 O2 (square) and Li4 Ti5 O12 coated-LiNi1/3 Co1/3 Mn1/3 O2 (circle) at a constant current density of 0.064 mA cm−2 in the voltage range between 2.5 and 4.4 V (vs. Li).
Fig. 6. Discharge curves of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cells using noncoated-LiNi1/3 Co1/3 Mn1/3 O2 (dashed line) and Li4 Ti5 O12 coatedLiNi1/3 Co1/3 Mn1/3 O2 (solid line). The cells were charged to 4.4 V (vs. Li) under a constant current density of 0.064 mA cm−2 and discharged at the current densities of 1.3 and 3.8 mA cm−2 .
current density of 0.064 mA cm−2 and discharged at current densities of 1.3 and 3.8 mA cm−2 . The discharge capacities of the cell using noncoated-LiNi1/3 Co1/3 Mn1/3 O2 at 1.3 and 3.8 mA cm−2 were 63 and 30 mAh g−1 , respectively. On the other hand, the cell using Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 exhibited the discharge capacities of 83 and 42 mAh g−1 at 1.3 and 3.8 mA cm−2 , respectively. The discharge capacity was increased and overpotential was decreased by the coating at high current densities over 1 mA cm−2 . In the cells for measuring rate performance, it was confirmed that the resistances R1 were the same in both cells. The coating techniques in the all-solid-state cell using LiNi1/3 Co1/3 Mn1/3 O2 are effective in improving rate performance because of decreasing the resistance R2 for the interfacial resistance of LiNi1/3 Co1/3 Mn1/3 O2 /Li2 S–P2 S5 . The similar effect on electrochemical property by coating was reported in all-solidstate batteries using LiCoO2 [20] and LiMn2 O4 [29]. In those cells, the coating layer acted as a buffer layer decreasing interfacial resistance during the initial charge process. In the In/Li2 S–P2 S5 /LiCoO2 cells, cross-sectional TEM–EDX observation demonstrated that an oxide coating layer on LiCoO2 suppressed the formation of an interfacial layer, which was caused by the structural and composition change at the interface of LiCoO2 /Li2 S–P2 S5 [21]. A structural change at the interface LiNi1/3 Co1/3 Mn1/3 O2 /Li2 S–P2 S5 after the initial charge process was observed by TEM–EDX analysis as shown later in Fig. 10. The coating effects in the cell using LiNi1/3 Co1/3 Mn1/3 O2 is thus estimated to be similar to the cell using LiCoO2 [20,21]. As described above, LiNi1/3 Co1/3 Mn1/3 O2 is a promising positive electrode material for the all-solid-state cell. We compared the electrochemical performance of LiNi1/3 Co1/3 Mn1/3 O2 with those of LiCoO2 , LiMn2 O4 and LiNiO2 to clarify the potential of LiNi1/3 Co1/3 Mn1/3 O2 in the all-solid-state cell. Commercial LiCoO2 and LiMn2 O4 particles were used and their electrochemical properties were reported in our previous papers [18,22]. LiNiO2 was prepared by the solution method for the comparisons. Fig. 7 depicts the charge–discharge curves of the all-solid-state In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/LiNiO2 cell. The cell was charged and discharged between 3.0 and 4.2 V (vs. Li) at a current density of 0.064 mA cm−2 . The cell showed the initial charge capacity of about 120 mAh g−1 . A large irreversible capacity was observed and the initial discharge capacity was about 50 mAh g−1 . However, the cell retained the reversible capacity of 48 mAh g−1 for 200 cycles. Fig. 8 shows the impedance spectra of the charged all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following
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Fig. 7. Charge–discharge curves of the In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/ LiNiO2 cell at a constant current density of 0.064 mA cm−2 in the voltage range between 3.0 and 4.2 V (vs. Li).
electrodes: (a) LiCoO2 , (b) LiMn2 O4 , (c) LiNi1/3 Co1/3 Mn1/3 O2 and (d) LiNiO2 . The cells were charged up to 4.2 V (vs. Li) at 0.064 mA cm−2 . Fig. 8(a )–(c ) are respectively magnified impedance spectra of (a)–(c). The all-solid-state cells using LiCoO2 and LiMn2 O4 exhibited almost the same interfacial resistance of about 70 and the interfacial resistance of cell using LiNi1/3 Co1/3 Mn1/3 O2 was slightly larger than them. On the contrary, the cell using LiNiO2 showed the resistance lager than 400 as shown in Fig. 8(d). In the previous reports [18,21,23], it was suggested that the interfacial resistance would include several resistive components such as space charge layer, reaction product like transition metal sulfide, and charge transfer process. We investigated the activation energy of the interfacial resistance in the all-solid-state cells using LiNi1/3 Co1/3 Mn1/3 O2 and LiNiO2 . Fig. 9
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Fig. 9. Temperature dependence of reciprocal interfacial resistance for the cells using LiNi1/3 Co1/3 Mn1/3 O2 (circle) and LiNiO2 (square).
shows the temperature dependences of reciprocal interfacial resistance (R2 ) of the all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/LiNi1/3 Co1/3 Mn1/3 O2 and In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/LiNiO2 . The activation energies were calculated from the Arrhenius equation expressed as: 1 = A exp R2
−E a
RT
where Ea is the activation energy, A is the pre-exponential factor, and R is the gas constant. The activation energies of the interfacial resistance in the cells using LiNi1/3 Co1/3 Mn1/3 O2 and LiNiO2 were 50 and 51 kJ mol−1 , respectively. It was reported that the cells using LiCoO2 and LiMn2 O4 showed the activation energies of 61 and
Fig. 8. Impedance spectra of the charged cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following electrodes: (a) LiCoO2 , (b) LiMn2 O4 , (c) LiNi1/3 Co1/3 Mn1/3 O2 and (d) LiNiO2 . The cells were charged up to 4.2 V (vs. Li) at 0.064 mA cm−2 . The spectra (a )–(c ) are magnified impedance spectra (a)–(c), respectively.
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Fig. 10. (a) Cross-sectional HAADF-STEM image of the interface between a LiNi1/3 Co1/3 Mn1/3 O2 particle and the solid electrolyte in the composite electrode. EDX spectrum measured at the marked position in (a) is shown in (b).
50 kJ mol−1 , respectively [20,29]. Although the main factor of these activation energies was not obvious at the present stage, LiCoO2 showed higher activation energy than the other positive electrodes. To confirm interfacial structure between LiNi1/3 Co1/3 Mn1/3 O2 and the solid electrolyte, the cross-section of the composite LiNi1/3 Co1/3 Mn1/3 O2 electrode layer after charging to 4.4 V (vs. Li) was observed by scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF)–STEM image of the composite electrode after charging is shown in Fig. 10(a). The elemental point analyses using energy dispersive X-ray (EDX) spectroscopy were carried out on the solid electrolyte in the vicinity of the interface (marked with circle). Fig. 10(b) shows the EDX spectrum measured at the position marked in Fig. 10(a). The elements of S, P and O derived from the 80Li2 S·19P2 S5 ·1P2 O5 solid electrolyte were detected. The impurity of starting materials of the solid electrolyte and contamination during sample preparation and transfer process for TEM observations would also contribute to the presence of oxygen. The elements of Si, W and Ga are also detected. The thin sample for TEM observations was prepared by cutting out from the sample pellet on a silicon sample holder with FIB milling. Before the FIB milling, tungsten was coated on the pellet to protect the thin sample from Ga ion beam. Therefore these elements were derived from residues for FIB milling. The transition metal elements were observed on the solid electrolyte in the vicinity of the LiNi1/3 Co1/3 Mn1/3 O2 particle. The diffusion of the transition metal elements suggested that an interfacial layer, which would influence on the impedance spectra, was formed in the vicinity of LiNi1/3 Co1/3 Mn1/3 O2 after initial charge process. The similar experiment was carried out in the cell using LiNiO2 electrode after initial charge process to 4.2 V (vs. Li). The Ni diffusion from LiNiO2 into the sulfide solid electrolytes was also detected. In the previous reports [18,21], the Co of LiCoO2 and Mn of LiMn2 O4 diffused into the solid electrolyte after initial charging process. It was found that the composition change of active materials and solid electrolytes at the electrode–electrolyte interface occurred in any positive electrode materials shown above. Then we compared the electrochemical performance between the cells using 4 V class positive electrode materials. At the first stage, the first irreversible capacities of the all-solid-state cells were evaluated. Fig. 11 shows the first charge–discharge curves at the current density of 0.064 mA cm−2 in the all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following electrodes: LiCoO2 , LiMn2 O4 , LiNiO2 and LiNi1/3 Co1/3 Mn1/3 O2 . As described above, the LiNiO2 showed a large irreversible capacity of 65 mAh g−1 , which was equal to the 57% of the initial charge
capacity. A large irreversible capacity was also observed in the cell using LiNi1/3 Co1/3 Mn1/3 O2 . The irreversible capacity was about 50 mAh g−1 , which was equal to the 30% of the initial charge capacity. On the other hand, the cell using LiCoO2 and LiMn2 O4 showed smaller irreversible capacity of about 10 mAh g−1 . There were several reports related to the irreversible capacity of positive electrode materials [31–36]. It was reported that the irreversible capacity for the cells using the LiNiO2 active material was caused by the cation mixing of nickel atom in the lithium layer [31,33]. The presence of nickel in the lithium layer formed inactive domains during charge process and the inactive domains prevented lithium from intercalation into the lithium layer. It was also reported that the kinetic properties of the positive electrodes influenced on the irreversible capacity [34,36]. The decrease of lithium diffusivities within LiNi1/3 Co1/3 Mn1/3 O2 and LiNiO2 derivatives at the end of the discharge process caused the initial irreversible capacity. In our case, one possible reason for the irreversible capacity of the cell using LiNiO2 is the cation mixing in the LiNiO2 electrode. In contrast, the irreversible capacity of the cell using LiNi1/3 Co1/3 Mn1/3 O2 would be caused by the decrease of lithium diffusivity at the end of discharge process because the interfacial resistance was small as well as the cell using LiCoO2 and LiMn2 O4 after initial charging. The cycle stability was compared between four kinds of positive electrode in the all-solid-state cells. Fig. 12 shows the discharge
Fig. 11. The first charge–discharge curves at the current density of 0.064 mA cm−2 in the all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following electrodes: LiCoO2 (solid line), LiMn2 O4 (dashed line), LiNiO2 (dotted line) and LiNi1/3 Co1/3 Mn1/3 O2 (heavy solid line).
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Table 1 Summary of characteristics of positive electrode materials. Cut-off voltage/V (vs. Li) LiCoO2 LiMn2 O4 LiNiO2 LiNi1/3 Co1/3 Mn1/3 O2 a
2.5–4.2 3.0–4.6 3.0–4.2 2.5–4.4
1st discharge capacity in the all-solid-state cells/mAh g−1 (0.064 mA cm−2 )
Capacity in a cell using liquid electrolytes/mAh g−1
Average particle size/m
e− /S cm−1
DLi /cm2 s−1
110 55 50 115
140a 90a 168 [28] 160 [25]
10 4 <1,1–10 10
1.2 × 10−3 6.7 × 10−5 0.28 8.3 × 10−5
10−8 to 10−13 [37–40] 10−9 to 10−14 [41–43] 10−11 to 10−13 [44–46] 10−10 to 10−11 [47–49]
Data sheet obtained from Toda Kogyo Co.
capacities as a function of cycle number in the all-solid-state cells using LiCoO2 , LiMn2 O4 , LiNiO2 and LiNi1/3 Co1/3 Mn1/3 O2 at the current density of 0.064 mA cm−2 . The cells using LiCoO2 , LiMn2 O4 and LiNiO2 showed stable cycle properties during 100 cycles. Although the cell using LiNi1/3 Co1/3 Mn1/3 O2 showed the cycle performance for only 10 cycles, a better capacity retention was expected. The rate
Fig. 12. Cycle performance at the current density of 0.064 mA cm−2 in the all-solid-state cells In/LiCoO2 (circle), LiMn2 O4 (diamond), LiNiO2 (triangle) or LiNi1/3 Co1/3 Mn1/3 O2 (square).
performance was also compared. Fig. 13 shows the capacity retention of the all-solid-state cells as a function of current densities. The cells were charged at a constant current density of 0.064 mA cm−2 and discharged at different current densities of 0.064–6.4 mA cm−2 . The discharge capacities at each current density were normalized by the capacity at 0.064 mA cm−2 . LiCoO2 showed the best rate performance and the capacity retention at 6.4 mA cm−2 was more than 60% of the capacity at 0.064 mA cm−2 . Table 1 shows the electrical, electrochemical and morphological properties of LiCoO2 , LiMn2 O4 , LiNiO2 and LiNi1/3 Co1/3 Mn1/3 O2 particles [37–49]. The all-solid-state cell using LiCoO2 showed the initial discharge capacity of 110 mAh g−1 , which was 79% of capacity in the cells using liquid electrolytes. The good electrochemical performance of LiCoO2 is due to its high electronic conductivity and high lithium ion diffusion coefficient. LiNiO2 exhibited the lowest capacity retention at all the current densities because the lithium diffusion coefficient of LiNiO2 is not so high and LiNiO2 showed the high interfacial resistance in the all-solid-state cell. The higher interfacial resistance would be caused by the inactive domain and the reaction product formed at the electrode–electrolyte interface as mentioned above. LiMn2 O4 showed the better rate performance from 0.13 to 2.6 mA cm−2 than LiNi1/3 Co1/3 Mn1/3 O2 . However, the capacity retention of LiMn2 O4 at 6.4 mA cm−2 was almost the same as that of LiNi1/3 Co1/3 Mn1/3 O2 . There was no difference of the electronic conductivity between LiMn2 O4 and LiNi1/3 Co1/3 Mn1/3 O2 . In addition, the lithium diffusion coefficient of LiMn2 O4 seems to be almost the same as that of LiNi1/3 Co1/3 Mn1/3 O2 on the basis of previous reports [43,48]. It indicated that the difference of the rate performance between LiMn2 O4 and LiNi1/3 Co1/3 Mn1/3 O2 would be attributed to the morphology of the particles. The particle size of LiNi1/3 Co1/3 Mn1/3 O2 was larger than that of LiMn2 O4 as shown in Fig. 1. The capacity retention of LiNi1/3 Co1/3 Mn1/3 O2 was thus lower than those of LiCoO2 and LiMn2 O4 in spite of no difference of interfacial resistance between these materials. However, the initial reversible capacity of LiNi1/3 Co1/3 Mn1/3 O2 at 0.064 mA cm−2 was larger than that of LiCoO2 . If the rate capability of LiNi1/3 Co1/3 Mn1/3 O2 is enhanced, LiNi1/3 Co1/3 Mn1/3 O2 will be the most promising candidate of the all-solid-state cells using Li2 S–P2 S5 solid electrolytes for the large-scale applications. Applying smaller particles probably improves the rate performance of the all-solid-state cell using LiNi1/3 Co1/3 Mn1/3 O2 . 4. Conclusions
Fig. 13. Capacity retention as a function of current density in the all-solid-state cells In/LiCoO2 (circle), LiMn2 O4 (diamond), LiNiO2 (triangle) or LiNi1/3 Co1/3 Mn1/3 O2 (square). The cells were charged at a constant current density of 0.064 mA cm−2 and discharged at different current densities of 0.064–6.4 mA cm−2 . The discharge capacities at each current density were normalized by the capacity at 0.064 mA cm−2 .
The all-solid-state cells using LiNi1/3 Co1/3 Mn1/3 O2 positive electrode were fabricated and the first reversible capacity was about 115 mAh g−1 at the current density of 0.064 mA cm−2 . The reversible capacity of about 110 mAh g−1 was retained after 10 cycles. The interfacial resistance between the LiNi1/3 Co1/3 Mn1/3 O2 electrode and solid electrolyte was observed in the impedance spectrum of the all-solid-state cell charged to 4.4 V (vs. Li) and the diffusion of transition metal elements was detected by the TEM observations with EDX analyses. The coating of LiNi1/3 Co1/3 Mn1/3 O2 particles with Li4 Ti5 O12 was effective for decreasing the interfacial resistance and improving the electro-
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chemical properties such as rate performance. The cell using LiNi1/3 Co1/3 Mn1/3 O2 showed the larger reversible capacity at 0.064 mA cm−2 than the cells using LiCoO2 , LiMn2 O4 and LiNiO2 . If the rate performance is increased by improving the morphology, the positive electrode LiNi1/3 Co1/3 Mn1/3 O2 will be the most promising candidate for the all-solid-state cells using Li2 S-P2 S5 solid electrolytes. References [1] A.A. Schneider, K.O. Fester, T. Kuder, J. Power Sources 5 (1975) 651. [2] C.C. Liang, J. Electrochem. Soc. 120 (1973) 1289. [3] K. Kanehori, K. Matsumoto, K. Miyauchi, T. Kudo, Solid State Ionics 9–10 (1983) 1445. [4] C. Julien, S. Saikh, M. Balkanski, Mater. Sci. Eng. B 14 (1992) 121. [5] F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, Solid State Ionics 177 (2006) 2721. [6] A. Hayashi, T. Ohtomo, F. Mizuno, K. Tadanaga, M. Tatsumisago, Electrochem. Commun. 5 (2003) 701. [7] M. Tatsumisago, A. Hayashi, J. Non-Crystalline Solids 354 (2008) 1411. [8] M. Nagao, H. Kitaura, A. Hayashi, M. Tatsumisago, J. Power Sources 189 (2009) 672. [9] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodnough, Mater. Res. Bull. 15 (1980) 783. [10] J.N. Reimers, J.R. Dahn, J. Electrochem. Soc. 139 (1992) 2091. [11] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972. [12] J.M. Tarascon, M. Armand, Nature 414 (2001) 359. [13] T. Ohzuku, M. Kitagawa, T. Hirai, J. Electrochem. Soc. 137 (1990) 769. [14] Y. Xia, M. Yoshio, J. Electrochem. Soc. 143 (1996) 825. [15] M.G.S.R. Thomas, W.I.P. David, J.B. Goodenough, P. Groves, Mater. Res. Bull. 20 (1985) 1137. [16] J.R. Dahn, U.V. Sacken, C.A. Michal, Solid State Ionics 44 (1990) 87. [17] K. Iwamoto, N. Aotani, K. Takada, S. Kondo, Solid State Ionics 79 (1995) 288. [18] H. Kitaura, A. Hayashi, K. Tadanaga, M. Tatsumisago, J. Electrochem. Soc. 157 (2010) A407. [19] K. Takada, N. Aotani, K. Iwamoto, S. Kondo, Solid State Ionics 79 (1995) 284. [20] A. Sakuda, H. Kitaura, A. Hayashi, K. Tadanaga, M. Tatsumisago, J. Electrochem. Soc. 156 (2009) A27.
[21] A. Sakuda, A. Hayashi, M. Tatsumisago, Chem. Mater. 22 (2010) 949. [22] F. Mizuno, S. Hama, A. Hayashi, K. Tadanaga, T. Minami, M. Tatsumisago, Chem. Lett. (2002) 1244. [23] N. Ohta, K. Takada, L. Zhang, R. Ma, M. Osada, T. Sasaki, Adv. Mater. 18 (2006) 2226. [24] T. Ohzuku, Y. Makimura, Chem. Lett. 1 (2001) 642. [25] K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, Electrochim. Acta 48 (2002) 145. [26] I. Belharouak, Y.K. Sun, J. Liu, K. Amine, J. Power Sources 123 (2003) 247. [27] T. Ohtomo, F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, J. Power Sources 146 (2005) 715. [28] M.Y. Song, R. Lee, J. Power Sources 111 (2002) 97. [29] H. Kitaura, A. Hayashi, K. Tadanaga, M. Tatsumisago, Solid State Ionics, submitted for publication. [30] K. Takada, N. Aotani, K. Iwamoto, S. Kondo, Solid State Ionics 86–88 (1996) 877. [31] H. Arai, S. Okada, Y. Sakurai, J.-I. Yamaki, Solid State Ionics 95 (1997) 275. [32] S.S. Zhang, K. Xu, T.R. Jow, Electrochem. Solid-State Lett. 5 (2002) A92. [33] J.P. Pérès, C. Delmas, A. Rougier, M. Broussely, F. Perton, P. Biensan, P. Willmann, J. Phys. Chem. Solids 57 (1996) 1057. [34] J.R. Mueller-Neuhaus, R.A. Dunlap, J.R. Dahn, J. Electrochem. Soc. 147 (2000) 3598. [35] J. Choi, A. Manthiram, Electrochem. Solid-State Lett. 8 (2005) C102. [36] S.-H. Kang, W.-S. Yoon, K.-W. Nam, X.-Q. Yang, D.P. Abraham, J. Mater. Sci. 43 (2008) 4701. [37] K. Dokko, M. Mohamedi, Y. Fujita, T. Itoh, M. Nishizawa, M. Umeda, I. Uchida, J. Electrochem. Soc. 148 (2001) A422. [38] H. Xia, L. Lu, G. Ceder, J. Power Sources 159 (2006) 1422. [39] S.B. Tang, M.O. Lai, L. Lu, J. Alloy Comp. 449 (2008) 300. [40] J. Xie, N. Imanishi, T. Matsumura, A. Hirano, Y. Takeda, O. Yamamoto, Solid State Ionics 179 (2008) 362. [41] K. Dokko, M. Mohamedi, M. Umeda, I. Uchida, J. Electrochem. Soc. 150 (2003) A425. [42] K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, J. Mater. Chem. 13 (2003) 106. [43] J. Xie, K. Kohno, T. Matsumura, N. Imanishi, A. Hirano, Y. Takeda, O. Yamamoto, Electrochim. Acta 54 (2008) 376. [44] G.X. Wang, S. Zhong, D.H. Bradhurst, S.X. Dou, H.K. Liu, Solid State Ionics 116 (1999) 271. [45] O. Yamada, M. Ishikawa, M. Morita, Electrochim. Acta 44 (1999) 1607. [46] J. Cho, H. Jung, Y.C. Park, G.B. Kim, H.S. Lim, J. Electrochem. Soc. 147 (2000) 15. [47] K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, J. Electrochem. Soc. 151 (2004) A1324. [48] Y. Idemoto, T. Matsui, Solid State Ionics 179 (2008) 625. ´ [49] M. Gozu, K. Swierczek, Janina Molenda, J. Power Sources 194 (2009) 38.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical performance of all-solid-state lithium secondary batteries with Li–Ni–Co–Mn oxide positive electrodes Hirokazu Kitaura, Akitoshi Hayashi, Kiyoharu Tadanaga, Masahiro Tatsumisago ∗ Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 25 December 2009 Received in revised form 21 July 2010 Accepted 24 July 2010 Available online 13 August 2010 Keywords: All-solid-state battery Electrode–electrolyte interface Rate performance Lithium nickel cobalt manganese oxide Li2 S–P2 S5 solid electrolyte
a b s t r a c t LiNi1/3 Co1/3 Mn1/3 O2 was applied as a promising material to the all-solid-state lithium cells using the 80Li2 S·19P2 S5 ·1P2 O5 (mol%) solid electrolyte. The cell showed the first discharge capacity of 115 mAh g−1 at the current density of 0.064 mA cm−2 and retained the reversible capacity of 110 mAh g−1 after 10 cycles. The interfacial resistance was observed in the impedance spectrum of the all-solid-state cell charged to 4.4 V (vs. Li) and the transition metal elements were detected on the solid electrolyte in the vicinity of LiNi1/3 Co1/3 Mn1/3 O2 by the TEM observations with EDX analyses. The electrochemical performance was improved by the coating of LiNi1/3 Co1/3 Mn1/3 O2 particles with Li4 Ti5 O12 film. The interfacial resistance was decreased and the discharge capacity was increased from 63 to 83 mAh g−1 at 1.3 mA cm−2 by the coating. The electrochemical performance of LiNi1/3 Co1/3 Mn1/3 O2 was compared with that of LiCoO2 , LiMn2 O4 and LiNiO2 in the all-solid-state cells. The rate capability of LiNi1/3 Co1/3 Mn1/3 O2 was lower than that of LiCoO2 . However, the reversible capacity of LiNi1/3 Co1/3 Mn1/3 O2 at 0.064 mA cm−2 was larger than that of LiCoO2 , LiMn2 O4 and LiNiO2 . © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Recently, environmental and energetic concerns have been growing and electric vehicles and hybrid electric vehicles have been developed. Lithium-ion batteries are attracting much attention as power sources for eco-cars owing to their great merits compared with other batteries. However, safety issues of lithium-ion batteries have to be solved to build large-scale battery packs. All-solid-state lithium secondary batteries using inorganic solid electrolytes have attracted attention because they have an advantage in safety [1–4]. To construct all-solid-state lithium batteries, the solid electrolytes with high lithium ion conductivities are required. Glass–ceramic electrolytes in the system Li2 S–P2 S5 have high lithium ion conductivities of about 10−3 S cm−1 and were used as solid electrolytes in the all-solid-state lithium batteries [5–8]. Researching electrode materials suitable for all-solid-state batteries is an important work to achieve the performance needed for eco-cars. Layered lithium cobalt oxide (LiCoO2 ) is the most common positive electrode material in lithium ion secondary batteries [9–11]. LiCoO2 has been used in commercially available lithium ion batteries over the past decade because LiCoO2 shows excellent electrochemical performance. However, its high cost and relatively small capacity (140–150 mAh g−1 ) make it difficult to be used in
∗ Corresponding author. Tel.: +81 72 2549331; fax: +81 72 2549331. E-mail address: [email protected] (M. Tatsumisago). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.07.066
large-scale batteries, and many researchers have explored better active materials. Spinel lithium manganese oxide (LiMn2 O4 ), layered lithium nickel oxide (LiNiO2 ) and their derivatives have been often studied as strong candidates for the 4 V class positive electrode materials of lithium-ion batteries [12–16]. The manganese or nickel compounds with abundance and environmental friendliness are suitable materials for the increase of production of large-scale batteries. The cells using LiMn2 O4 show higher charge–discharge voltage of around 4.0 V (vs. Li) and higher safety even at overcharge condition compared with the cell using LiCoO2 . However, the practical capacity of LiMn2 O4 (100–140 mAh g−1 ) is slightly smaller than that of LiCoO2 and the cell using LiMn2 O4 show capacity degradation during cycling at 60 ◦ C. LiNiO2 is isostructural with layered structure of LiCoO2 and was found to give high capacity (180–200 mAh g−1 ). However, there are several drawbacks such as difficulty to prepare stoichiometric compound, low thermal stability and poor capacity retention during long cycling. These 4 V class positive electrode materials were used in the all-solid-state lithium secondary batteries with sulfide-based solid electrolytes [17–19]. We applied LiCoO2 and LiMn2 O4 to the allsolid-state cells with Li2 S–P2 S5 solid electrolytes and investigated their charge–discharge performance and interfacial properties [18,20,21]. The all-solid-state cells using LiCoO2 exhibited a long cycle performance with the reversible capacity of over 100 mAh g−1 under a current density of 0.064 mA cm−2 [22]. Although the LiMn2 O4 showed relatively small capacity of 55 mAh g−1 in the allsolid-state cells, a long cycle life was achieved. These cells showed
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Fig. 1. FE-SEM images of (a) LiNi1/3 Co1/3 Mn1/3 O2 , (b) LiCoO2 , (c)LiMn2 O4 , (d) LiNiO2 particles and (e) Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 .
similar impedance behaviors after initial charge process; the resistance attributable to the interface between positive electrode and solid electrolyte was observed [18,20]. The electrode–electrolyte interface was also analyzed by the TEM–EDX measurements and the diffusion of the transition metal elements from active materials into the solid electrolyte was detected in the vicinity of active material particles [18,21]. Ohta et al. demonstrated that the interfacial resistance between LiCoO2 and the Li3.25 Ge0.25 P0.75 S4 electrolyte was decreased by oxide coatings [23]. Surface coatings of active material particles were also effective for decreasing the interfacial resistance in the all-solid-state cells using the Li2 S–P2 S5 electrolytes [20]. As a result, the rate performance of the all-solid-state cells was improved. Lithium nickel–cobalt–manganese oxide (LiNi1/3 Co1/3 Mn1/3 O2 ) was first proposed by Ohzuku et al. and larger capacity (160 mAh g−1 in the voltage range 2.5–4.4 V and 200 mAh g−1 in the voltage range 2.5–4.6 V) and higher thermal stability were reported [24–26]. From the viewpoints of both economy and environment conservation, less cobalt content of LiNi1/3 Co1/3 Mn1/3 O2 compared with LiCoO2 is one of the advantages of this material. In the present study, all-solid-state cells using LiNi1/3 Co1/3 Mn1/3 O2 active materials were assembled and their electrochemical properties were investigated. The 80Li2 S·19P2 S5 ·1P2 O5 (mol%) glass–ceramic with a high lithium ion conductivity was used as a solid electrolyte [27]. The interfacial analyses between the LiNi1/3 Co1/3 Mn1/3 O2 electrode and the Li2 S–P2 S5 electrolyte were carried out by the impedance mea-
surements and transmission electron microscopy. The surface of LiNi1/3 Co1/3 Mn1/3 O2 particles was coated with the Li4 Ti5 O12 film to improve the rate performance. Lithium nickel oxide (LiNiO2 ) was also applied to the all-solid-state cells. The electrochemical performance of the cells using LiNi1/3 Co1/3 Mn1/3 O2 was compared with those using LiCoO2 , LiMn2 O4 and LiNiO2 positive electrodes. 2. Experimental Lithium nickel–cobalt–manganese oxide particles (LiNi1/3 Co1/3 Mn1/3 O2 , Nippon Chemical Industrial Co.) were used as an active material for all-solid-state cells. It was confirmed by using X-ray diffraction (XRD-6000; Shimadzu Co.) and fieldemission scanning electron microscope (FE-SEM/S4500; Hitachi, Ltd.) measurements that the particles had layered ␣-NaFeO2 structure and that their diameter was about 10 m as shown in Fig. 1(a). LiCoO2 and LiMn2 O4 particles were supplied from Toda Kogyo Co. and their morphology was shown in Fig. 1(b) and (c), respectively. LiNiO2 was synthesized by the solution method [28]. Lithium nitrate (LiNO3 , Wako Pure Chemical Industries, Ltd.), nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, Wako Pure Chemical Industries, Ltd.) and citric acid (C2 H5 O7 , Wako Pure Chemical Industries, Ltd.) were used as starting materials. LiNO3 and Ni(NO3 )2 ·6H2 O were dissolved in distilled water at 70–80 ◦ C. The aqueous solution was mixed with citric acid aqueous solution and was stirred for 24 h. After drying at 80 ◦ C, calcination processes at 600 ◦ C for 5 h in air
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and 800 ◦ C for 10 h in O2 atmosphere were performed. The obtained powder was identified as LiNiO2 by the XRD measurement and the particles had a large size distribution from 1 to 10 m as shown in Fig. 1(d). The amorphous lithium titanate (Li4 Ti5 O12 ) was coated on the LiNi1/3 Co1/3 Mn1/3 O2 particles by the sol–gel method [23,29]. Li4 Ti5 O12 sol was prepared from lithium ethoxide and titanium tetraisopropoxide. The LiNi1/3 Co1/3 Mn1/3 O2 particles were put into the sol and the sol was dried with stirring. The LiNi1/3 Co1/3 Mn1/3 O2 particles with the Li4 Ti5 O12 gel film were heated at 350 ◦ C for 30 min. Finally, we obtained the LiNi1/3 Co1/3 Mn1/3 O2 particles coated with Li4 Ti5 O12 film. Fig. 1(e) shows the SEM image of the Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 particles. The morphology such as size and shape of the particles was similar to that of the noncoated particles as shown in Fig. 1(a). The thickness of coating layer with less than 1 nm was estimated from the BET surface area of LiNi1/3 Co1/3 Mn1/3 O2 particles and weight of Li4 Ti5 O12 sol. The electronic conductivities of active materials were measured by applying a constant d.c. voltage to the pellets. A pair of stainless-steel plates as non-blocking electrodes was attached to both faces of the pellet. The electronic conductivities were calculated from the current values and applied voltage by using a Potentiostat/Galvanostat device (Solartron 1287 coupled with Solartron 1260). All-solid-state electrochemical cells were assembled as follows. The 80Li2 S·19P2 S5 ·1P2 O5 (mol%) glass was prepared by the mechanochemical reaction process [27]. The glass–ceramic electrolyte was prepared by crystallization of the glass. The obtained glass–ceramic was used as a solid electrolyte for all-solid-state cells. A composite electrode was prepared as a working electrode by mixing an active material (38.5 wt.%), the solid electrolyte (57.7 wt.%), and vapor grown carbon fiber (3.8 wt.%) powders. The composite electrode (10 mg) and the solid electrolyte powder (80 mg) were set in a polycarbonate tube (˚ = 10 mm) and were then pressed under 360 MPa. Then In foil (thickness, 100 m) was put on the solid electrolyte layer as a counter and reference electrode; the pressure of 240 MPa was applied to the three-layered pellet and then relieved. The estimated thickness of the solid electrolyte layer and the composite electrode layer were about 600 m and less than 100 m. Finally, the pellet sandwiched by two stainless-steel rods as current collectors was obtained. All processes were performed in an Ar-filled glove box. Electrochemical tests were conducted at 25 ◦ C in an Ar atmosphere using charge–discharge measuring devices (BTS-2004; Nagano Co.). Electrochemical impedance measurements were performed for the electrochemical cells, which were charged to the different depths at the constant current conditions and then aged for several hours. An impedance analyzer (Solartron 1287 coupled with Solartron 1260) was used and a small perturbation voltage of 50 mV in the frequency range of 1–10 MHz was applied for the measurements. The interface between an active material particle and the solid electrolyte in the composite electrode layer was analyzed using a transmission electron microscope (TEM, JEM2100, JEOL). Focused ion beam (FIB) milling was done for sample-preparation for cross-sectional TEM observation. Elemental point analysis for the cross-section of the composite electrode layer was carried out using energy dispersive X-ray spectroscopy (EDX, JED-2300T, JEOL).
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Fig. 2. Charge–discharge curves of the In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/ cell (solid line) and the In/80Li2 S·19P2 S5 ·1P2 O5 LiNi1/3 Co1/3 Mn1/3 O2 glass–ceramic/Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 cell (dashed line) at a constant current density of 0.064 mA cm−2 in the voltage range between 2.5 and 4.4 V (vs. Li).
versus the Li–In electrode as a counter electrode; the vertical axis on the right side denotes the potential versus Li electrode, which was calculated based on the potential difference between Li–In and Li [30]. The cell was charged and discharged between 2.5 and 4.4 V (vs. Li) at a current density of 0.064 mA cm−2 . The cell showed the typical charge–discharge curves of LiNi1/3 Co1/3 Mn1/3 O2 . The initial charge capacity of about 160 mAh g−1 was obtained and the cell showed the reversible capacity of about 115 mAh g−1 . Fig. 3 shows the impedance spectrum of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cell charged up to 4.4 V (vs. Li) at 0.064 mA cm−2 . The cell showed the three resistance components in the high (R1 ), middle (R2 ) and low (R3 ) frequency regions. The impedance spectrum was resemble to that observed in the In/LiCoO2 and In/LiMn2 O4 cells using Li2 S–P2 S5 solid electrolyte after initial charging [18,20]. We analyzed the impedance spectra in the cells using LiCoO2 and LiMn2 O4 in the previous study and we thus presumed that the resistance of R1 , R2 and R3 in the allsolid-state cell using LiNi1/3 Co1/3 Mn1/3 O2 were ascribed to the solid electrolyte layer, LiNi1/3 Co1/3 Mn1/3 O2 electrode-solid electrolyte interface and counter electrode, respectively. In order to confirm that R2 was attributed to the electrode–electrolyte interface, Li4 Ti5 O12 coating layer was introduced into the interface between LiNi1/3 Co1/3 Mn1/3 O2 and solid electrolyte. Fig. 4 shows the impedance spectra of the all-solid-state cells using (a) noncoated and (b) Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 after the first charge process to 4.4 V (vs. Li). The difference of R1 between both the cells is ascribed to the
3. Results and discussion Fig. 2 depicts the charge–discharge curves (solid line) of the all-solid-state In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/noncoated LiNi1/3 Co1/3 Mn1/3 O2 cell. In the voltage profiles of the all-solidstate cells, the vertical axis on the left side denotes the potential
Fig. 3. Impedance spectrum of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cell after charging up to 4.4 V (vs. Li) at 0.064 mA cm−2 . Three resistance components in the high (>100 kHz), middle (peak top frequency of 250 Hz) and low (<10 Hz) frequency regions are denoted by R1 , R2 and R3 , respectively.
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Fig. 4. Impedance spectra of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cells using (a) noncoated-LiNi1/3 Co1/3 Mn1/3 O2 and (b) Li4 Ti5 O12 coated-LiNi1/3 Co1/3 Mn1/3 O2 charged up to 4.4 V (vs. Li) at 0.064 mA cm−2 .
use of different batches of the solid electrolyte, which exhibited somewhat different conductivity; we confirmed that the R1 resistances were consistent with the resistances estimated from the conductivity of each solid electrolyte layer in both cells. The R2 resistances of the cells using noncoated and Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 were about 200 and 100 , respectively. The interfacial resistance was decreased by the coating and then the coating effects on the electrochemical performance were investigated. The charge–discharge curves (dashed line) of the all-solid-state In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/Li4 Ti5 O12 coated LiNi1/3 Co1/3 Mn1/3 O2 cell were also depicted in Fig. 2. The reversible capacity increased by the coating. The cycle performance of the all-solid-state cells using noncoated and Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 at the current density of 0.064 mA cm−2 was shown in Fig. 5. Both the cells retained the reversible capacity after second cycle and the all-solid-state cell using Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 retained the capacity of about 120 mAh g−1 after 10 cycles. Fig. 6 shows the discharge curves of the all-solid-state cells using noncoated and Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 in dependence of current density. The cells were charged to 4.4 V (vs. Li) at a constant
Fig. 5. Cycle performance of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cells using noncoated-LiNi1/3 Co1/3 Mn1/3 O2 (square) and Li4 Ti5 O12 coated-LiNi1/3 Co1/3 Mn1/3 O2 (circle) at a constant current density of 0.064 mA cm−2 in the voltage range between 2.5 and 4.4 V (vs. Li).
Fig. 6. Discharge curves of the all-solid-state In/LiNi1/3 Co1/3 Mn1/3 O2 cells using noncoated-LiNi1/3 Co1/3 Mn1/3 O2 (dashed line) and Li4 Ti5 O12 coatedLiNi1/3 Co1/3 Mn1/3 O2 (solid line). The cells were charged to 4.4 V (vs. Li) under a constant current density of 0.064 mA cm−2 and discharged at the current densities of 1.3 and 3.8 mA cm−2 .
current density of 0.064 mA cm−2 and discharged at current densities of 1.3 and 3.8 mA cm−2 . The discharge capacities of the cell using noncoated-LiNi1/3 Co1/3 Mn1/3 O2 at 1.3 and 3.8 mA cm−2 were 63 and 30 mAh g−1 , respectively. On the other hand, the cell using Li4 Ti5 O12 -coated LiNi1/3 Co1/3 Mn1/3 O2 exhibited the discharge capacities of 83 and 42 mAh g−1 at 1.3 and 3.8 mA cm−2 , respectively. The discharge capacity was increased and overpotential was decreased by the coating at high current densities over 1 mA cm−2 . In the cells for measuring rate performance, it was confirmed that the resistances R1 were the same in both cells. The coating techniques in the all-solid-state cell using LiNi1/3 Co1/3 Mn1/3 O2 are effective in improving rate performance because of decreasing the resistance R2 for the interfacial resistance of LiNi1/3 Co1/3 Mn1/3 O2 /Li2 S–P2 S5 . The similar effect on electrochemical property by coating was reported in all-solidstate batteries using LiCoO2 [20] and LiMn2 O4 [29]. In those cells, the coating layer acted as a buffer layer decreasing interfacial resistance during the initial charge process. In the In/Li2 S–P2 S5 /LiCoO2 cells, cross-sectional TEM–EDX observation demonstrated that an oxide coating layer on LiCoO2 suppressed the formation of an interfacial layer, which was caused by the structural and composition change at the interface of LiCoO2 /Li2 S–P2 S5 [21]. A structural change at the interface LiNi1/3 Co1/3 Mn1/3 O2 /Li2 S–P2 S5 after the initial charge process was observed by TEM–EDX analysis as shown later in Fig. 10. The coating effects in the cell using LiNi1/3 Co1/3 Mn1/3 O2 is thus estimated to be similar to the cell using LiCoO2 [20,21]. As described above, LiNi1/3 Co1/3 Mn1/3 O2 is a promising positive electrode material for the all-solid-state cell. We compared the electrochemical performance of LiNi1/3 Co1/3 Mn1/3 O2 with those of LiCoO2 , LiMn2 O4 and LiNiO2 to clarify the potential of LiNi1/3 Co1/3 Mn1/3 O2 in the all-solid-state cell. Commercial LiCoO2 and LiMn2 O4 particles were used and their electrochemical properties were reported in our previous papers [18,22]. LiNiO2 was prepared by the solution method for the comparisons. Fig. 7 depicts the charge–discharge curves of the all-solid-state In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/LiNiO2 cell. The cell was charged and discharged between 3.0 and 4.2 V (vs. Li) at a current density of 0.064 mA cm−2 . The cell showed the initial charge capacity of about 120 mAh g−1 . A large irreversible capacity was observed and the initial discharge capacity was about 50 mAh g−1 . However, the cell retained the reversible capacity of 48 mAh g−1 for 200 cycles. Fig. 8 shows the impedance spectra of the charged all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following
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Fig. 7. Charge–discharge curves of the In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/ LiNiO2 cell at a constant current density of 0.064 mA cm−2 in the voltage range between 3.0 and 4.2 V (vs. Li).
electrodes: (a) LiCoO2 , (b) LiMn2 O4 , (c) LiNi1/3 Co1/3 Mn1/3 O2 and (d) LiNiO2 . The cells were charged up to 4.2 V (vs. Li) at 0.064 mA cm−2 . Fig. 8(a )–(c ) are respectively magnified impedance spectra of (a)–(c). The all-solid-state cells using LiCoO2 and LiMn2 O4 exhibited almost the same interfacial resistance of about 70 and the interfacial resistance of cell using LiNi1/3 Co1/3 Mn1/3 O2 was slightly larger than them. On the contrary, the cell using LiNiO2 showed the resistance lager than 400 as shown in Fig. 8(d). In the previous reports [18,21,23], it was suggested that the interfacial resistance would include several resistive components such as space charge layer, reaction product like transition metal sulfide, and charge transfer process. We investigated the activation energy of the interfacial resistance in the all-solid-state cells using LiNi1/3 Co1/3 Mn1/3 O2 and LiNiO2 . Fig. 9
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Fig. 9. Temperature dependence of reciprocal interfacial resistance for the cells using LiNi1/3 Co1/3 Mn1/3 O2 (circle) and LiNiO2 (square).
shows the temperature dependences of reciprocal interfacial resistance (R2 ) of the all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/LiNi1/3 Co1/3 Mn1/3 O2 and In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/LiNiO2 . The activation energies were calculated from the Arrhenius equation expressed as: 1 = A exp R2
−E a
RT
where Ea is the activation energy, A is the pre-exponential factor, and R is the gas constant. The activation energies of the interfacial resistance in the cells using LiNi1/3 Co1/3 Mn1/3 O2 and LiNiO2 were 50 and 51 kJ mol−1 , respectively. It was reported that the cells using LiCoO2 and LiMn2 O4 showed the activation energies of 61 and
Fig. 8. Impedance spectra of the charged cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following electrodes: (a) LiCoO2 , (b) LiMn2 O4 , (c) LiNi1/3 Co1/3 Mn1/3 O2 and (d) LiNiO2 . The cells were charged up to 4.2 V (vs. Li) at 0.064 mA cm−2 . The spectra (a )–(c ) are magnified impedance spectra (a)–(c), respectively.
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Fig. 10. (a) Cross-sectional HAADF-STEM image of the interface between a LiNi1/3 Co1/3 Mn1/3 O2 particle and the solid electrolyte in the composite electrode. EDX spectrum measured at the marked position in (a) is shown in (b).
50 kJ mol−1 , respectively [20,29]. Although the main factor of these activation energies was not obvious at the present stage, LiCoO2 showed higher activation energy than the other positive electrodes. To confirm interfacial structure between LiNi1/3 Co1/3 Mn1/3 O2 and the solid electrolyte, the cross-section of the composite LiNi1/3 Co1/3 Mn1/3 O2 electrode layer after charging to 4.4 V (vs. Li) was observed by scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF)–STEM image of the composite electrode after charging is shown in Fig. 10(a). The elemental point analyses using energy dispersive X-ray (EDX) spectroscopy were carried out on the solid electrolyte in the vicinity of the interface (marked with circle). Fig. 10(b) shows the EDX spectrum measured at the position marked in Fig. 10(a). The elements of S, P and O derived from the 80Li2 S·19P2 S5 ·1P2 O5 solid electrolyte were detected. The impurity of starting materials of the solid electrolyte and contamination during sample preparation and transfer process for TEM observations would also contribute to the presence of oxygen. The elements of Si, W and Ga are also detected. The thin sample for TEM observations was prepared by cutting out from the sample pellet on a silicon sample holder with FIB milling. Before the FIB milling, tungsten was coated on the pellet to protect the thin sample from Ga ion beam. Therefore these elements were derived from residues for FIB milling. The transition metal elements were observed on the solid electrolyte in the vicinity of the LiNi1/3 Co1/3 Mn1/3 O2 particle. The diffusion of the transition metal elements suggested that an interfacial layer, which would influence on the impedance spectra, was formed in the vicinity of LiNi1/3 Co1/3 Mn1/3 O2 after initial charge process. The similar experiment was carried out in the cell using LiNiO2 electrode after initial charge process to 4.2 V (vs. Li). The Ni diffusion from LiNiO2 into the sulfide solid electrolytes was also detected. In the previous reports [18,21], the Co of LiCoO2 and Mn of LiMn2 O4 diffused into the solid electrolyte after initial charging process. It was found that the composition change of active materials and solid electrolytes at the electrode–electrolyte interface occurred in any positive electrode materials shown above. Then we compared the electrochemical performance between the cells using 4 V class positive electrode materials. At the first stage, the first irreversible capacities of the all-solid-state cells were evaluated. Fig. 11 shows the first charge–discharge curves at the current density of 0.064 mA cm−2 in the all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following electrodes: LiCoO2 , LiMn2 O4 , LiNiO2 and LiNi1/3 Co1/3 Mn1/3 O2 . As described above, the LiNiO2 showed a large irreversible capacity of 65 mAh g−1 , which was equal to the 57% of the initial charge
capacity. A large irreversible capacity was also observed in the cell using LiNi1/3 Co1/3 Mn1/3 O2 . The irreversible capacity was about 50 mAh g−1 , which was equal to the 30% of the initial charge capacity. On the other hand, the cell using LiCoO2 and LiMn2 O4 showed smaller irreversible capacity of about 10 mAh g−1 . There were several reports related to the irreversible capacity of positive electrode materials [31–36]. It was reported that the irreversible capacity for the cells using the LiNiO2 active material was caused by the cation mixing of nickel atom in the lithium layer [31,33]. The presence of nickel in the lithium layer formed inactive domains during charge process and the inactive domains prevented lithium from intercalation into the lithium layer. It was also reported that the kinetic properties of the positive electrodes influenced on the irreversible capacity [34,36]. The decrease of lithium diffusivities within LiNi1/3 Co1/3 Mn1/3 O2 and LiNiO2 derivatives at the end of the discharge process caused the initial irreversible capacity. In our case, one possible reason for the irreversible capacity of the cell using LiNiO2 is the cation mixing in the LiNiO2 electrode. In contrast, the irreversible capacity of the cell using LiNi1/3 Co1/3 Mn1/3 O2 would be caused by the decrease of lithium diffusivity at the end of discharge process because the interfacial resistance was small as well as the cell using LiCoO2 and LiMn2 O4 after initial charging. The cycle stability was compared between four kinds of positive electrode in the all-solid-state cells. Fig. 12 shows the discharge
Fig. 11. The first charge–discharge curves at the current density of 0.064 mA cm−2 in the all-solid-state cells In/80Li2 S·19P2 S5 ·1P2 O5 glass–ceramic/one of the following electrodes: LiCoO2 (solid line), LiMn2 O4 (dashed line), LiNiO2 (dotted line) and LiNi1/3 Co1/3 Mn1/3 O2 (heavy solid line).
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Table 1 Summary of characteristics of positive electrode materials. Cut-off voltage/V (vs. Li) LiCoO2 LiMn2 O4 LiNiO2 LiNi1/3 Co1/3 Mn1/3 O2 a
2.5–4.2 3.0–4.6 3.0–4.2 2.5–4.4
1st discharge capacity in the all-solid-state cells/mAh g−1 (0.064 mA cm−2 )
Capacity in a cell using liquid electrolytes/mAh g−1
Average particle size/m
e− /S cm−1
DLi /cm2 s−1
110 55 50 115
140a 90a 168 [28] 160 [25]
10 4 <1,1–10 10
1.2 × 10−3 6.7 × 10−5 0.28 8.3 × 10−5
10−8 to 10−13 [37–40] 10−9 to 10−14 [41–43] 10−11 to 10−13 [44–46] 10−10 to 10−11 [47–49]
Data sheet obtained from Toda Kogyo Co.
capacities as a function of cycle number in the all-solid-state cells using LiCoO2 , LiMn2 O4 , LiNiO2 and LiNi1/3 Co1/3 Mn1/3 O2 at the current density of 0.064 mA cm−2 . The cells using LiCoO2 , LiMn2 O4 and LiNiO2 showed stable cycle properties during 100 cycles. Although the cell using LiNi1/3 Co1/3 Mn1/3 O2 showed the cycle performance for only 10 cycles, a better capacity retention was expected. The rate
Fig. 12. Cycle performance at the current density of 0.064 mA cm−2 in the all-solid-state cells In/LiCoO2 (circle), LiMn2 O4 (diamond), LiNiO2 (triangle) or LiNi1/3 Co1/3 Mn1/3 O2 (square).
performance was also compared. Fig. 13 shows the capacity retention of the all-solid-state cells as a function of current densities. The cells were charged at a constant current density of 0.064 mA cm−2 and discharged at different current densities of 0.064–6.4 mA cm−2 . The discharge capacities at each current density were normalized by the capacity at 0.064 mA cm−2 . LiCoO2 showed the best rate performance and the capacity retention at 6.4 mA cm−2 was more than 60% of the capacity at 0.064 mA cm−2 . Table 1 shows the electrical, electrochemical and morphological properties of LiCoO2 , LiMn2 O4 , LiNiO2 and LiNi1/3 Co1/3 Mn1/3 O2 particles [37–49]. The all-solid-state cell using LiCoO2 showed the initial discharge capacity of 110 mAh g−1 , which was 79% of capacity in the cells using liquid electrolytes. The good electrochemical performance of LiCoO2 is due to its high electronic conductivity and high lithium ion diffusion coefficient. LiNiO2 exhibited the lowest capacity retention at all the current densities because the lithium diffusion coefficient of LiNiO2 is not so high and LiNiO2 showed the high interfacial resistance in the all-solid-state cell. The higher interfacial resistance would be caused by the inactive domain and the reaction product formed at the electrode–electrolyte interface as mentioned above. LiMn2 O4 showed the better rate performance from 0.13 to 2.6 mA cm−2 than LiNi1/3 Co1/3 Mn1/3 O2 . However, the capacity retention of LiMn2 O4 at 6.4 mA cm−2 was almost the same as that of LiNi1/3 Co1/3 Mn1/3 O2 . There was no difference of the electronic conductivity between LiMn2 O4 and LiNi1/3 Co1/3 Mn1/3 O2 . In addition, the lithium diffusion coefficient of LiMn2 O4 seems to be almost the same as that of LiNi1/3 Co1/3 Mn1/3 O2 on the basis of previous reports [43,48]. It indicated that the difference of the rate performance between LiMn2 O4 and LiNi1/3 Co1/3 Mn1/3 O2 would be attributed to the morphology of the particles. The particle size of LiNi1/3 Co1/3 Mn1/3 O2 was larger than that of LiMn2 O4 as shown in Fig. 1. The capacity retention of LiNi1/3 Co1/3 Mn1/3 O2 was thus lower than those of LiCoO2 and LiMn2 O4 in spite of no difference of interfacial resistance between these materials. However, the initial reversible capacity of LiNi1/3 Co1/3 Mn1/3 O2 at 0.064 mA cm−2 was larger than that of LiCoO2 . If the rate capability of LiNi1/3 Co1/3 Mn1/3 O2 is enhanced, LiNi1/3 Co1/3 Mn1/3 O2 will be the most promising candidate of the all-solid-state cells using Li2 S–P2 S5 solid electrolytes for the large-scale applications. Applying smaller particles probably improves the rate performance of the all-solid-state cell using LiNi1/3 Co1/3 Mn1/3 O2 . 4. Conclusions
Fig. 13. Capacity retention as a function of current density in the all-solid-state cells In/LiCoO2 (circle), LiMn2 O4 (diamond), LiNiO2 (triangle) or LiNi1/3 Co1/3 Mn1/3 O2 (square). The cells were charged at a constant current density of 0.064 mA cm−2 and discharged at different current densities of 0.064–6.4 mA cm−2 . The discharge capacities at each current density were normalized by the capacity at 0.064 mA cm−2 .
The all-solid-state cells using LiNi1/3 Co1/3 Mn1/3 O2 positive electrode were fabricated and the first reversible capacity was about 115 mAh g−1 at the current density of 0.064 mA cm−2 . The reversible capacity of about 110 mAh g−1 was retained after 10 cycles. The interfacial resistance between the LiNi1/3 Co1/3 Mn1/3 O2 electrode and solid electrolyte was observed in the impedance spectrum of the all-solid-state cell charged to 4.4 V (vs. Li) and the diffusion of transition metal elements was detected by the TEM observations with EDX analyses. The coating of LiNi1/3 Co1/3 Mn1/3 O2 particles with Li4 Ti5 O12 was effective for decreasing the interfacial resistance and improving the electro-
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