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Cycle life tests of commercial 18650-type lithium-ion cells were conducted at temperatures of 0°C, 25°C, and 45°C and current rates of 1 C and 2 C. Surprisingly, the discharge capacity rapidly decreased at 0°C regardless of the current rate. On the other hand, the discharge capacity decreased faster at 45°C than at 25°C for a rate of 1 C, and this decrease was faster at 25°C than 45°C for a rate of 2 C. Electrodes and electrolytic solutions of the degraded cells were characterized by several analytical methods. X-ray photoelectron spectroscopy revealed differences between solid electrolyte interface components according to test temperature. The carbonate ratio was high at low temperature while the phosphate ratio was high at elevated temperature. The observed cell degradations were caused by several factors exhibiting various temperature and current rate dependencies.

In this study, we conducted systematic storage life tests using commercial 18650-type lithium-ion batteries with Li 1−x Mn 2 O 4 -and Li 1−y Ni 0.5 Co 0.2 Mn 0.3 O 2 -blended cathodes at four temperatures (0°C, 25°C, 45°C, and 60°C) and six state of charges (SOCs) (0%, 40%, 60%, 70%, 80%, and 100%). We conducted a non-disassemble dV/dQ curve analysis to understand the cell-degradation mechanism. Cathode/anode reaction region slips mainly caused cell capacity fading and were accelerated by temperature and SOC. Cathode degradation was also observed at specific SOCs of 60% and 70% at 45°C and 60°C. The proposed method calculated the Li composition of Li 1−x Mn 2 O 4 from the dV/dQ curve and the cell charge capacity on adjusting the SOC value. The Li composition obtained was such that manganese from Li 1−x Mn 2 O 4 was easily eluted at SOCs of 60% and 70%. Even at fixed SOC definition voltages, the Li compositions were observed to vary as degradation proceeded; this variation affected the degradation rate.

Introduction There is an increasing demand for the performance and lifetime predictions of lithium-ion batteries under real operating conditions for their application into electric vehicles and energy storage systems. In order to establish a degradation model, the degradation factors, such as decomposition of electrolyte, formation of solid electrolyte interface (SEI), Li metal deposition, etc..., should be evaluated. In this paper, we conducted cycle tests using commercial 18650-type lithium ion cells at the conditions of three temperatures (0 °C, 25 °C, and 45 °C) and two current rates (1 C and 2 C). The electrodes and electrolytes of the degraded cells were also investigated by the several analytical methods. Experimental The active materials of cathode and anode are Li(Ni1/3Mn1/3Co1/3)O2 and graphite, respectively. The electrolyte is composed of EC, PC, EMC, and DMC, containing LiPF6. The discharge capacities and internal resistances were periodically measured at 25 °C during the cycle tests. To understand the degradation mechanism, we disassembled the degraded cells and took out the electrodes and the electrolytic solution. The obtained electrolytic solutions were investigated by 1H- and 19F-NMR, and GC-MS. The electrodes were investigated by ICP-MS, XRD, XPS, 7Li-NMR, and SEM. Moreover, the solid electrolyte interfaces (SEIs) of the electrodes were extracted by solvent and investigated by 1H- and 19F-NMR. Results and discussion Fig. 1 shows the discharge capacity measured at 25 °C vs. cumulative discharge capacity plots during the cycle life tests. Surprisingly, the discharge capacities rapidly decreased at 0 °C conditions regardless of the cycle rates. As for 25 °C and 45 °C conditions, the discharge capacity decreased faster at 45 °C at 1 C rate, while discharge capacity decreased faster at 25 °C at 2 C rate. Fig. 2 shows the increase in the internal resistances measured at 25 °C, and again, the internal resistances increased rapidly at 0 °C, and the internal resistance at 25 °C increased faster than that at 45 °C at 2 C rate. At 1 C rate, the internal resistance at 25 °C increased faster than that at 45 °C before the drastic increase at around 1500 Ah. The electrolyte analyses revealed that the decomposition of LiPF6, which causes increase in internal resistance, occur faster at higher temperature conditions at both 1 C and 2 C cycle tests. As a consequence, the amount of inorganic metal fluoride SEI investigated by 19F-NMR was larger at higher temperature conditions. 1H-NMR showed that the organic SEI formations also progressed at high temperature. These SEI formations consume active lithium for charge/discharge, and hence, the discharge capacities decrease. On the contrary, 7Li-NMR of anode showed that the metal Li tended to deposit at low temperature and high cycle rate. The surface SEM images drastically changed at high cycle rate, indicating the SEI formations of the anode proceed fast. In addition, XPS spectra showed that the component of the SEIs was different, that is, the ratio of carbonates was high at low temperature, and the ratio of phosphate was high at high temperature. In summary, the observed cell degradations were caused by the several degradation factors which had different temperature/cycle rate dependences.

Introduction               Lithium-ion battery life is expected to increase, and the need for degradation analyses has grown [1]. Among the degradation mechanism, the change in the electrode surface state has significant influence on lithium-ion battery performance. In this study, a long-term cycle test was conducted using commercial 18650-type lithium ion cells for a current rate of 1 C at 25 °C. The electrode surface state was investigated by X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES), and transmission electron microscopy (TEM).  Experimental               The cycle test was performed using a 18650-type commercial lithium-ion cell equipped with a Li(Ni1/3Mn1/3Co1/3)O2 cathode and a graphite anode. After cycle test completion, the cell was analyzed at a state-of-charge of 50% (= 3.694 V) by electrochemical impedance spectroscopy at 25 °C. The degraded single electrode property was measured by preparing a half cell against lithium metal. The electrode surface state was characterized by XPS and HAXPES. HAXPES measurements were performed at the BL46XU beamline of SPring-8. The cathode surface state was also investigated by TEM. The electrolytic solution was analyzed by 1H- and 19F-NMR.  Results and discussion               During the cycle test, the discharge capacity decreased gradually and then drastically above 1500 cycles. The capacity retention measured at 1 C reached below 10% after 1700 cycles. Nyquist plots before and after cycle test showed an increase in electrolyte and charge transfer resistances (Fig. 1).               To understand the degradation mechanism, the degraded cell was disassembled after the cell was discharged at C/20, and their electrodes and electrolytic solution were removed under argon atmosphere. Half cells were prepared, and measured electrodes were characterized. After the first cycle, the degraded cathode exhibited a coulombic efficiency of 186%, indicating that it was partially charged when the cell was opened. Conversely, the degraded anode displayed a coulombic efficiency of about 100%, showing it was fully discharged. This mismatch between cathode and anode state-of-charge may explain the decrease in cell capacity. In addition, charge/discharge curves in the second cycle showed a capacity loss of about 20% at the cathode compared to initial condition.               XPS and HAXPES spectra of Mn 2p, Co 2p, and Ni 2p were measured to investigate the cathode surface state. XPS and HAXPES detection depths are approximately 6 and 30 nm, respectively. No spectral changes were observed for Mn 2p 1/2 and Co 2p 1/2. Figure 2 shows the XPS and HAXPES spectra of Ni 2p 1/2. Ni 2p 1/2 XPS peak shifted toward higher energies (around 1.9 eV), indicative of the higher valence state of Ni at the cathode. Ni 2p 1/2 HAXPES peak only slightly shifted (around 0.7 eV) as a potential result of the oxidation of Ni2+, consistent with the partially charged state of the cathode. The difference between XPS and HAXPES suggests that the cycle test changed the cathode surface state. The structural properties of the cathode surface were evaluated by TEM. The lattice fringes of the layered rock-salt Li(Ni1/3Mn1/3Co1/3)O2 structure disappeared from the active material surface, and the electron diffraction pattern of the surface was unclear. These results indicate that amorphization occurred at the cathode surface (about 3 nm), augmenting the charge transfer resistance. Moreover, electrolyte analysis revealed that LiPF6was decomposed, explaining the increase in electrolyte resistance.               In summary, the long-term cycle test revealed that the capacity loss stemmed from the shift of cathode and anode reaction regions and cathode degradation. Furthermore, the increase in impedance spectrum resulted from cathode surface amorphization and LiPF6decomposition.  Acknowledgment               Synchrotron radiation experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Institute (JASRI) (Proposal No. 2013A1234, 2014A1558, 2014B1015, and 2014B1594) [1] T. Matsuda et al. 226th ECS meeting A5-343 Figure 1