Low Temperature Electrochemical Properties of Li[NixCoyMn1-x-y]O2 Cathode Materials for Lithium Batteries

Lithium-ion batteries are used in various areas as rechargeable power sources due to their high energy density and power capability. They are also being considered as power sources for hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEV). Layered Li[Ni_1-xM_x]O₂ (M = transition metal) has been investigated as a candidate cathode material for the above-mentioned applications owing to its relatively low cost, low toxicity, and high reversible capacity. However, lithium ion batteries exhibit inferior temperature performance such as low capacity and power capability, in particular in extreme conditions such as at a temperature of −20^◦C. For instance, the Li[Ni_0.8Co_0.15Al_0.05]O₂ cell tested between 3.0 and 4.3 V at a 0.2 C-rate (40 mA g⁻¹) a capacity loss of 14% after only 20 cycles at −10^◦C. In the case of the olivine LiFePO₄ (the theoretical capacity is 170 mAh g⁻¹), the discharge capacity is approximately 90 mAh g⁻¹ at −20^◦C. In addition, a commercial 18650 Li-ion battery with a LiCoO₂ cathode retained only 5% of its energy density at −40^◦C, compared to the density at 20^◦C. To fulfill the requirements in those harsh conditions, the cathode materials used should have a high specific energy and good cycleability even at low temperatures.

 Ni-rich Li[Ni_xCo_yMn_1-x-y]O₂ materials are promising due to their high capacity that exceeds 200 mAh g⁻¹, which results from the Ni^2+/3+/4+ and Co^3+/4+ redox couples. In contrast, the high amount of Mn in the transition metal layer would not be favored in terms of capacity, due to the fact that the average oxidation state is tetravalent so that it does not solve the electrochemical reaction but instead provides structural stability upon cycling. However, the low temperature electrode performances of Ni-rich Li[Ni_xCo_yMn_1-x-y]O₂ are rarely reported in literature. Capacity fade is mainly related with slow transport of Li⁺ ions at low temperatures. Except this fact, capacity fade mechanism has not been elucidated further. This motivates us to explore optimal compositions of cathode that operate at extreme conditions. Since diffusion is governed by particle size of active material, we limit the particle sizes of active materials to 10–11 μm in diameter to minimize the particle size effect of the Li[Ni_xCo_yMn_1-x-y]O₂ (x:y:1-x-y=1/3:1/3:1/3, 0.5:0.2:0.3, 0.6:0.2:0.2, 0.70:0.15:0.15, 0.8:0.1:0.1, and 0.85:0.075:0.075 hereafter defined as x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85, respectively). Here, we introduce the electrochemical properties of Li[Ni_xCo_yMn_1-x-y]O₂ at low temperatures (0 ∼ −20^◦C) and suggest possible factors that affect the low temperature properties.

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