Whether attempting to eliminate parasitic Li metal plating on graphite (and other Li-ion anodes) or enabling stable, uniform Li metal formation in 'anode-free' Li battery configurations, the detection and characterization (morphology, microstructure, chemistry) of Li that cannot be reversibly cycled is essential to understand the behavior and degradation of rechargeable batteries. In this review, various approaches used to detect and characterize the formation of Li in batteries are discussed. Each technique has its unique set of advantages and limitations, and works towards solving only part of the full puzzle of battery degradation. Going forward, multimodal characterization holds the most promise towards addressing two pressing concerns in the implementation of the next generation of batteries in the transportation sector (viz. reducing recharging times and increasing the available capacity per recharge without sacrificing cycle life). Such characterizations involve combining several techniques (experimental-and/or modeling-based) in order to exploit their respective advantages and allow a more comprehensive view of cell degradation and the role of Li metal formation in it. It is also discussed which individual techniques, or combinations thereof, can be implemented in real-world battery management systems on-board electric vehicles for early detection of potential battery degradation that would lead to failure.
Strong, solid polymer electrolyte ion gels, with moduli in the MPa range, a capacitance of 2 μF/cm(2), and high ambient ionic conductivities (>1 × 10(-3) S/cm), all at room temperature, have been prepared from butyl-N-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide (PYR14TFSI) and methyl cellulose (MC). These properties are particularly attractive for supercapacitor applications. The ion gels are prepared by codissolution of PYR14TFSI and MC in N,N-dimethylformamide (DMF), which after heating and subsequent cooling form a gel. Evaporation of DMF leave thin, flexible, self-standing ion gels with up to 97 wt % PYR14TFSI, which have the highest combined moduli and ionic conductivity of ion gels to date, with an excellent electrochemical stability window (5.6 V). These favorable properties are attributed to the immiscibility of PYR14TFSI in MC, which permits the ionic conductivity to be independent of the MC at low MC content, and the in situ formation of a volume spanning network of semicrystalline MC nanofibers, which have a high glass transition temperature (Tg = 190 °C) and remain crystalline until they degrade at 300 °C.
Nanocomposite electrolytes have been prepared from mixtures of two polyoctahedral silsesquioxanes (POSS) nanomaterials, each with a SiO 1.5 core and eight side groups. POSS-PEG 8 has eight polyethylene glycol side chains that have low glass transition (T g ) and melt (T m ) temperatures and POSS-benzyl 7 (BF 3 Li) 3 is a Janus-like POSS with hydrophobic phenyl groups and −Si−O−BF 3 Li ionic groups clustered on one side of the SiO 1.5 cube. The electron-withdrawing POSS cage and BF 3 groups enable easy dissociation of the Li + . In the presence of polar POSS-PEG 8 , the hydrophobic phenyl rings of POSS-benzyl 7 (BF 3 Li) 3 aggregate and crystallize, forming a biphasic morphology, in which the phenyl rings form the structural phase and the POSS-PEG 8 forms the conductive phase. The −Si−O−BF 3 − Li + groups of POSS-benzyl 7 (BF 3 Li) 3 are oriented toward the polar POSS-PEG 8 phase and dissociate so that the Li + cations are solvated by the POSS-PEG 8 . The nonvolatile nanocomposite electrolytes are viscous liquids that do not flow under their own weight. POSS-PEG 8 /POSS-benzyl 7 (BF 3 Li) 3 at O/Li = 16/1 has a conductivity of σ = 2.5 × 10 −4 S/cm at 30 °C, which is 17 times greater than that of POSS-PEG 8 /LiBF 4 , and a low activation energy (E a ∼ 3−4 kJ/mol); σ = 1.6 × 10 −3 S/cm at 90 °C and 1.5 × 10 −5 S/cm at 10 °C. The lithium ion transference number was t Li + = 0.50 ± 0.01, as a result of the reduced mobility of the large, bulky anion, and the system exhibited low interfacial resistance that stabilized after 3 days (both at 80 °C).
Fast charging of
batteries for electric vehicles is seen as one
of the most direct ways to enhance adoption. Currently, fast charging
is limited by increased cell aging, which is primarily driven by Li
plating and degradation of cathode materials. Here, using combined
sets of experimental and computational analysis and a suite of different
charge protocols, we begin to examine the interplay between failure
mode, cell designs, and ultimately aging mechanisms. Slight variation
in cell design and the subsequent impacts that charge protocols have
on aging can create distinct cell-to-cell variation. As little as
2% difference in porosity change at the cell negative electrode during
cycling due to early Li metal plating has been found to alter the
aging pathway and either accelerate or inhibit the loss due to Li
plating.
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