Impact of Dry Particle Fusion Coating of Tungsten Oxide on Ni-Based Positive Electrode Materials for Li-Ion Batteries

Abstract

Ni-rich layered transition metal(TM) oxides LiNixTM1-xO2 (TM = Al, Mn, Mg, etc) are attractive for the positive electrode of Li-ion batteries because they have high specific and volumetric capacity 1–3 However, most Ni-rich layered transition metal oxides show poor capacity retention when cycled above 4.14 V vs Li/Li+, even though capacity retention when cycled below 4.14 V is excellent. Solving this mystery and improving capacity retention when Ni-rich layered transition metal oxides are required to deliver high specific capacity is of great importance.Tungsten doping has been reported to improve capacity retention of layered positive electrode materials when charged above 4.14 V. Ryu et al. found that the phase transitions were suppressed for tungsten doped layered oxide materials.4 Tungsten doping in Li[Ni0.9Co0.09W0.01]O2 (NCW) created secondary particles composed of thin directionally elongated needle-like primary particles. These particle morphology changes were claimed to be the reason for greatly improved capacity retention. The NCW cathode retained 92% of its initial capacity after 1000 cycles compared to only 63% for NCA.5 Microcrack formation by anisotropic volume expansion was found to be suppressed in secondary particles having these nanosized needle-like primary particles.5,6 Dry particle fusion of Al2O3 on Ni(OH)2 followed by lithiation has been shown to be an effective method to improve the capacity retention of positive electrode materials.7 In this work, tungsten-containing Ni-rich electrode materials containing 0.5, 1 and 2% W (compared to Ni) were made by dry particle fusion of WO3 on Ni(OH)2 followed by lithiation. The W-containing Ni-rich layered cathode materials had considerably improved capacity retention in agreement with previous reports. Figure 1a shows a W EDS map of some particles of Ni(OH)2 with 2 mol% WO3 after lithiation which shows that W ends up uniformly distributed inside the particles. Figure 1b shows that Ni(OH)2 coated with 0.5, 1 and 2 mol% WO3 followed by lithiation had better capacity retention than that of LiNiO2. Of all the samples prepared, Ni(OH)2 coated with 1 mol% WO3, followed by lithiation, had the largest specific discharge capacity and the best capacity retention. Figure 1c shows the specific discharge capacity vs. cycle number of Ni(OH)2 coated with 1 mol% WO3, followed by lithiation and Ni(OH)2 coated with 3 wt% Al2O3, followed by lithiation (corresponding to 5.17 mol% Al) reported in our previous work.7 The cycling performances of these two materials match almost exactly, indicating that they are equivalent materials. TEM and synchrotron based analysis are on-going to hopefully resolve the question: “Where is the W in these materials? – In a second phase or substitutional for Ni in the layered structure. Full coin cell cycling performance will be presented showing good capacity retention to 4.28 V vs Li/Li+. dVdQ analysis will also be presented allowing the cell degradation mechanism to be better understood.Reference U. H. Kim, D. W. Jun, K. J. Park, Q. Zh...