High Energy Density Li-Ion Batteries with Atomic-Level Multi-Functional Modified LiCoO2 Cathode
Authors: Xing Weibing 
Published: February 2020

Transition metal oxides are commonly used as cathode materials for state-of-the-art (SOA) Li-ion batteries. Owing to possessing a redox potential of ~ 4V (vs. Li/Li+), they are usually referred as 4V cathode materials. Among them, LiCoO2 (LCO) has been the most frequently used in commercial Li-ion batteries [1]. LCO has a layered structure with hexagonal symmetry where lithium ion (Li+) can reversibly be extracted via two-dimensional pathways [2]. The highly oxidizing Co4+/3+ redox couple provides a cell voltage of ~ 4V vs. Li/Li+. Although LCO has a theoretical specific capacity of 270 mAh/g, corresponding to x = 0 in LixCoO2 or ~ 5V vs. Li/Li+, only 50% of the LCO theoretical capacity can be practically achieved in commercial Li-ion cells, i.e. x » 0.5 in LixCoO2, corresponding to ~140 mAh/g or ~ 4.2V vs. Li/Li+. This is because further Li+ extraction or delithiation from pristine LCO, corresponding to x < 0.5 in LixCoO2 or > 4.2V vs. Li/Li+, will lead to a: (i) large concentration of Co4+ ions that are unstable (Co4+ dissolution) and (ii) large change in lattice parameter c accompanied by phase transitions that cause structure degradation [3]. In addition, conventional carbonate solvent-based Li-ion electrolytes are unstable (oxidative decomposition) when operating at high voltages, e.g., > 4.6V vs. Li/Li+. These factors can lead to accelerated battery capacity fade when attempting to gain more capacity (e.g., > 140 mAh/g) from pristine (unstabilized) LCO.

Here we present a novel approach to develop a high voltage, high energy density LCO (HV-LCO) with robust cycle life performance. We employ atomic-level coatings on LCO to provide novel multifunctional and/or enhanced performance. Data projections for a resultant battery system using HV-LCO exceed performance metrics for electric drive vehicle batteries, with a competitive cost per energy density.

Figure 1 shows scanning electron microscopy (SEM) images at 50Kx (magnification) for (a) pristine LCO and (b) atomic-level, metal fluoride coated LCO powder particles. Figure 1b shows a uniformly distributed, nano-sized metal fluoride coating on LCO, which is in clear contrast to the pristine LCO sample without ALD coating (Figure 1a).

We evaluated the electrochemical cycle stability of coated and baseline LCO cathode samples against lithium metal anodes in coin cells (Li/LCO half coin cells) between 3V and various upper voltages, at C/2 rate and at room temperature (RT). When cycled between 3 – 4.6V (representing a 100% state-of-charge, SOC), the ALD coated LCO delivered > 220 mAh/g specific capacity and > 900 Wh/kg specific energy (based on the cathode active mass), which are substantially higher than SOA cathodes for Li-ion batteries. For example, the specific energy of the HV-LCO represents more than 50% increase over LiNi0.8Co0.15Al0.05O2 (NCA), one of the highest specific energy cathode materials.

Figure 2 shows that, when cycled between 3 – 4.55V (representing > 90% SOC), the baseline LCO cell experienced a fast capacity fade vs. cycle number. On the other hand, the ALD metal fluoride, metal phosphate and lithium metal phosphate coated LCO cells demonstrated very little capacity fade when cycled to the high, upper voltage.

Our study shows that the nanoscale, multi-functional coatings on LCO afforded substantially increased energy density and much improved high voltage cycle stability for the resultant HV-LCO. Coupled with economically scalable coating processes, the significant energy density gain of the HV-LCO renders the cathode material cost competitive in terms of $/energy density over SOA cathodes. Hence, our HV-LCO represents a promising cathode material for high energy density, long cycle life and cost effective Li-ion batteries that is ideally suited for many high energy density-demanding applications such as, electric drive vehicles, consumer electronics and other military applications.

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Figure 1