Nanocrystalline Li x Mn2 − y  O 4 Cathodes for Solid‐State Thin‐Film Rechargeable Lithium Batteries

Abstract

The large number of recent papers on LiMn 2 O 4 spinel and related compounds attest to the intense interest in the attractive properties of these intercalation materials for possible use as reversible cathodes (positive electrodes) in lithium and lithium-ion batteries. Materials referred to as "defect spinels" include compositions extending from LiMn 2 O 4 to Li 2 Mn 4 O 9 and Li 4 Mn 5 O 12 . [1][2][3][4][5][6][7][8][9][10][11] The chemistry of these compounds is both complex and sensitive to the particular processing conditions, but recent work helps to clarify the stable phase equilibria. [8][9][10] In our laboratory, crystalline thin-film lithium manganese oxide cathodes have been fabricated by electron-beam evaporation and sputtering followed by a high-temperature anneal. 12-15 These films, when incorporated into a thin-film solid-state lithium cell, have ϳ4 V capacities of 50-120 mAh/g and have been cycled many hundreds of times with little degradation. Others have also prepared crystalline thin films of ϳLiMn 2 O 4 by physical vapor deposition and anneal techniques. [16][17][18][19][20] They report comparable capacities and in one case good cycling, 17 but all of these cells used a liquid electrolyte making them subject to undesired cathode-electrolyte reactions, particularly at high voltages and temperatures.The goal of the work reported here was to deposit LiMn 2 O 4 thinfilm cathodes at modest temperatures <150ЊC in order to permit thinfilm battery construction onto lower-temperature substrate materials, battery stacks, and host devices. Magnetron sputter deposition was chosen for fabrication of the thin-film cathodes as this energetic deposition technique often enhances density, crystallinity, and adhesion of films at low temperatures. It became clear, however, that films deposited at low temperatures were poorly crystalline. 21 Nevertheless, upon optimization of the deposition parameters, we find that even the nanocystalline (n) Li x Mn 2Ϫy O 4 cathodes are useful for low-power, thin-film battery applications, due to their high energy density and exceptional rechargeability.This report describes the cycling properties of this cathode when incorporated into our all-solid-state, thin-film battery. 22 We show cycling results extending to thousands of cycles at 25 and 100ЊC and to cell potentials from 5.3 to 1.5 V. The specific capacity and cell polarization are discussed with respect to the film composition and proposed free energy of mixing diagrams. ExperimentalThe thin-film cathodes were deposited by radio frequency (rf) magnetron sputtering from 2 in. diam LiMn 2 O 4 ceramic disks. Several targets were purchased (Cerac); others were fabricated in-house by cold pressing and sintering (1200ЊC, air). Powders were synthesized at 600ЊC from Li 2 CO 3 and MnO 2 , as well as purchased. All targets were 70-80% of theoretical density, and their X-ray diffraction (XRD) pattern matched the file for LiMn 2 O 4 (no. 35-782). Targets were either 0.125 or 0.25 in. thick, several being bonded to 0.125 in. copper bac...

New Developments in Solid Electrolytes for Thin-Film Lithium Batteries

“…Recently, solid-state lithium secondary batteries have attracted much attention because the replacement of conventional liquid electrolyte with an inorganic solid electrolyte may improve the safety and reliability of lithium batteries utilizing high capacity lithium metal anodes (Jones and Akridge, 1992). There are two types of solid state electrolytes; one is thin-film electrolytes grown by RF sputtering (Bates et al, 1993;Bates et al, 2000a;Bates et al, 2000b;Dudney et al, 1999;Neudecker et al, 2000;Seo and Martin, 2011a, b, c;Yu et al, 1997) or PLD (Jin et al, 2000;Tabata et al, 1994) etc. and the other is bulk electrolytes fabricated by typically using melting processes.…”

“…and the other is bulk electrolytes fabricated by typically using melting processes. All-solid-state thin-film batteries using inorganic amorphous electrolytes such as LiPON have been reported and LiPON shows excellent cyclability, over 50,000 cycles, at room temperature (Bates et al, 1993;Bates et al, 2000a;Bates et al, 2000b;Dudney et al, 1999;Neudecker et al, 2000;Yu et al, 1997). However, bulk type batteries using bulk electrolytes have an advantage of improving cell capacity by the addition of large amounts of active materials to the cell.…”

“…These materials were not thin-films but bulk glass materials. In addition, their properties were not fully characterized until Oak Ridge National Laboratory (ORNL) reported LiPON thinfilms (Bates et al, 1993;Bates et al, 2000a;Bates et al, 2000b;Dudney, 2000;Dudney, 2005;Dudney et al, 1999;Neudecker et al, 2000;Yu et al, 1997). The LiPON thin-films were deposited using a high purity lithium phosphate, Li 3 PO 4 , target by RF magnetron sputtering technique in nitrogen atmosphere.…”

“…This crosslinking is believed to decrease the electrostatic energy of the overall network, allowing for faster ion conduction. Thin-film batteries comprised of Li-LiCoO 2 cells and Li-LiMn 2 O 4 cells have been fabricated using the LiPON electrolyte at ORNL (Bates et al, 1993;Bates et al, 2000a;Bates et al, 2000b;Dudney, 2000;Dudney, 2005;Dudney et al, 1999;Neudecker et al, 2000;Park et al, 2007;Yu et al, 1997). Nam et al reported LiPON using V 2 O 5 cathode materials (Jeon et al, 2001).…”

“…Recently, Bates and Dudney et al at Oak Ridge National Laboratory (ORNL) reported significant progress on LiPON-based thin-film batteries which were produced by an RF sputtering technique (Bates et al, 1993;Bates et al, 2000a;Bates et al, 2000b;Dudney et al, 1999;Wang et al, 1995a;Wang et al, 1995b;Wang et al, 1995c;Yu et al, 1997). In order to fabricate LiPON thin-film batteries, the metallic anode was produced by vacuum evaporation and anode and cathodes were produced by RF sputtering.…”