Microstructure and Electrochemical Performance of Rapidly Sintered LiCoO2 Cathodes (2022)

Lithium ion battery technology based upon liquid carbonate electrolytes and intercalation electrodes has achieved widespread commercial success. The batteries power mobile devices like cell phones, laptop computers and cordless power tools. They are used in hybrid and all electric vehicles to reduce greenhouse gas emissions and dependency on limited petrochemical resources. They have been demonstrated to stabilize electric grids at local and national levels under periods of high demand. Even with these remarkable achievements, improvements in energy density are sought in batteries for most applications. Lithium metal batteries are being pursued around the world to meet this need. Rapid, continuous, roll-to-roll sintering of alumina ceramic tapes was recently demonstrated at Corning Incorporated [1]. The process is based on the pioneering work of Ketcham et al [2]. The ceramic tape, because it is thin, is flexible and may be heated, sintered, and cooled back to room temperature in less than one hour without cracking. One application of this technology is for manufacture of thin, &lt;30 µm, solid electrolyte separators. Another is to enable novel, cathode-supported battery architectures. To that end, rapid sintering has been extended to lithium cobaltite (LCO). Continuous sintering and winding of LCO is shown in Fig. 1. The LCO cathode because it is sintered and self-supporting may be used to reduce the proportions of inactive components to dramatically increase energy density. Both composite and dense cathodes can be envisioned. One possible way of creating a solid-state battery with LCO ribbon is to capitalize on the LiPON solid electrolyte originally developed at ORNL for thin-film micro-batteries (TFMB’s) [3, 4]. The materials in TFMB’s have proven track records and attractive performance attributes. The LiPON solid electrolyte in contact with a lithium metal anode is resistant to dendrite formation even at current densities &gt;5 mA/cm²; a claim that cannot be made for lithium garnet and lithium phosphosulfide glass [5, 6].TFMB’s also have tremendous cycling longevity and retain more than 90% of capacity after over 10 4 charge-discharge cycles [7]. The challenge with the technology is the high cost associated with growth of the cathode by sputtering processes; thicknesses are limited to ~2-3 µm. As a consequence, TFMB’s are a good fit for applications where small size power sources are needed like smart cards, medical implants, RFID tags, and wireless sensing. A thicker, rapidly sintered cathode could address the cost-capacity limitation. The cathode for this architecture could be made in a dense, closed pore form or with 15-25% porosity to host and electrolyte. The later would be expected to offer greater rate performance due to shorter diffusion distances and greater surface area for charge transfer. Energy density of more than 1,500 Wh/L is theoretically achievable. The performance of cathode materials like LCO depends upon synthesis conditions, process history, design and microstructure. We explore the electrochemical attributes of free standing LCO cathodes with thickness of 15 to 80 µm and porosity ranging from closed (dense) to ~25%. Cathodes were tested in 2032 coin cells using Whatman glass fiber as the separator and 1 M LiPF 6 in 1:1 EC:DMC. Capacity of the cathodes regardless of microstructure is near the theoretical value. Lithium diffusivity of dense polycrystalline cathodes was characterized by galvanostatic intermittent titration and averages 1.13×10 -9 cm²/s over the potential window of 3.0 to 4.3 V. Surface XRD shows the grain texture to be nominally random. Approximately 50% capacity was available at C/2 at a 23 µm thickness. Resistance of porous cathodes decreased with perimeter-to-surface area, and effective resistances below 10 Ωcm² were observed. Cycling endurance of porous sintered cathodes with 2.4 mAh/cm² capacity was evaluated with lithium titanate working electrodes. More than 90% capacity was retained after 1000 cycles between 3.0 and 4.3 V vs Li at 3C rate. [1] C. Kim, et al, “5G mm Wave Patch Antenna on Multi-layered Alumina Ribbon Ceramic Substrates,” IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting, July 2020, pp. 65-66. [2] T.D. Ketcham, et al., US 5,089,455, Feb 18, 1992. [3] J.B. Bates, et al., Solid State Ionics , 53-56 (1992) 647-654. [4] J.B. Bates, et al., U.S. 5,561,004, Oct 10, 1996. [5] F.D. Han, et al., Nat. Energy , (2019) 1-10. [6] J.C. Li, et al., Adv. Energy. Mater. , 5 (2015) 1401408. [7] J. Xie, et al., Solid State Ionics , 178 (2008) 362-370. Figure 1