Synthesis and Electrochemical Performance of Nickel-Rich Layered-Structure LiNixCoyMnzO2 (x > 0.65) Cathode Materials Comprising Particles with Ni and Mn Full Concentration Gradients
Authors: Aurbach Doron , Erickson Evan Mark , Bouzaglo Hana , Sclar Hadar , Park Kang-Joon , Lim Byung-Beom , Schipper Florian , Ghanty Chandan , Grinblat Judith , Markovsky Boris , Sun Yang-Kook 
Published: February 2020

Layered structure transition metal oxides of the form LiNixCoyMnzO2  (x + y + z = 1), are commercially available lithium ion battery materials that provide capacities of 140 – 200 mAhrg-1  over long lifetimes, ca. 3000 cycles with 80 % capacity retention. The composition of these materials can be used to tailor specific properties such as capacity, cycle life, low impedance, toxicity or cost. In order to enhance capacity and average operating voltage fade, Ni rich materials (x ≥ 0.65), can be used, but surface Ni-oxide and hydroxide moieties are highly alkaline, tending to react strongly with the standard electrolyte solutions, what limits the electrodes’ cycle life. One method to increase cycle life of electrodes, comprising Ni-rich material, is to synthesize materials with a low Ni surface content and high Ni concentration at the core. It is possible to synthesize Li[NiCoMn]O2 materials with Ni and Mn concentration gradients. Many different gradient variables may be explored, for example thickness of core and shell, differences between surface and bulk concentrations and the structure of the  gradient, e.g. linear, exponential, single or multiphase. For our work, we have synthesized cathode materials utilizing the coprecipitation method, exploring first different annealing protocols to produce Ni-rich LiNi0.65Co0.08Mn0.27O2. After optimization of the annealing conditions, we have monitored the effect of metal chelation agent concentration on the electrochemical performance of the battery material. Then, we explore an even more nickel-rich gradient material, the LiNi0.7Co0.1Mn0.2O2, also introducing a two-phase gradient with two distinct Ni/Mn concentration slopes. The materials are cycled in both half cells (vs. Li) and full cells vs. graphite, to explore short term and long term effects, respectively. Electrochemical impedance spectroscopy is utilized to monitor the evolution of surface film impedance during cycling. High temperature (up to 60 oC) cycling and mean discharge voltage stability are studied, exhibiting the enhanced stabilizing effects of the Ni/Mn gradients. In addition, bulk and surface changes such as the alteration of the gradient structure after prolonged cycling is explored via TEM and electron diffraction measurements. The extent of transition metal ions dissolution is measured by chemical analysis of anodes, removed after cycling, on which the metals are partially reduced. Figure 1 depicts typical galvanostatic cycling results of LiNi0.65Co0.08Mn0.27O2 vs. graphite full cells at hard conditions: high rate and elevate temperature, demonstrating an advantage to the cathode material with gradient concentration.  The entire study have proven that for Ni rich Li[NiCoMn]O2 cathode materials, the concentration gradient enables a better performance compared to similar cathode materials with uniform concentration.

Figure 1: Galvanostatic cycling of  gradient (FCGT, red) and standard, non-gradient (CC, black) cathode materials at high rates (2C for 29 cycles, then 1 cycle at C/10, repeated), at 45 oC  vs. graphite full cells.

Figure 1