Abstract
Solid block polymer electrolytes are promising candidates for the development of high energy density rechargeable lithium metal based batteries. All solid-state batteries comprising lithium metal negative electrode and lithium iron phosphate (LiFePO4) composite positive electrode were assembled. A polystyrene-b-poly(ethylene oxide) (SEO) copolymer doped with a lithium salt was used as the electrolyte. After cycling the batteries, the reason for capacity fade and failure was determined by imaging the batteries using synchrotron hard X-ray microtomography. These experiments revealed partial delamination of the lithium foil and the block copolymer electrolyte layer. The void volume between the foil and electrolyte layer obtained after 40 to 90 cycles is comparable to volume change in the battery during one cycle. A simple model to account for the effect of delamination on current density in the battery is presented. Capacity fade and battery failures observed in our experiments are consistent with this model. No evidence of lithium dendrite formation was found. In contrast, cycled lithium-lithium symmetric cells with the same polymer electrolyte at the same current density failed due to dendrite formation. No evidence of delamination was found in these cells.
Highlights
Title Failure mode of lithium metal batteries with a block copolymer electrolyte analyzed by X-ray microtomography
For symmetric SEO electrolytes, i.e. copolymers wherein the volume fraction of the conducting phase is above 50%, the ionic conductivity has been shown to increase with poly(ethylene oxide) (PEO) chain length until it plateaus when the number-averaged molecular weight (Mn) of the PEO block exceeds 100 kg/mol.[32]
Batteries comprising a lithium metal negative electrode, a solid block copolymer electrolyte, and an LiFePO4 based positive electrode were cycled at 90◦C at selected C-rates to determine the Peukert plot for each battery
Summary
Title Failure mode of lithium metal batteries with a block copolymer electrolyte analyzed by X-ray microtomography. Solid polymer electrolytes are promising candidates for the development of high performance rechargeable batteries comprising a lithium (Li) metal electrode due to their chemical stability toward lithium and their mechanical resistance to dendrite growth.[1,2,3] After the pioneering work by Fenton et al.,[4] where poly(ethylene oxide) (PEO) laden with alkali metal salts was shown to possess good ionic conductivity, its application as a polymer electrolyte in a full cell was demonstrated by Armand et al.[5,6,7] PEO is a semi-crystalline polymer at room temperature. There have been very few studies of adhesion between solid electrolytes (polymers, ceramics, glasses) and electrodes.[41,42]
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