The lithium-ion battery is one of the most common battery types for all sorts of technical devices. It can be used to power cell phones, laptops, power tools or electric vehicles. It is small wonder, therefore, that businesses around the world invest billions to make this industry grow. For many years now, scientists have been searching for electrode materials and electrolytes that can help improve lithium-ion batteries so that they may offer greater energy storage capacities while lasting longer, costing less and being safer.
Now (2019), scientists at Argonne National Laboratory have designed a new lithium-ion battery with a unique electrolyte additive - a second salt containing any one of several doubly or triply-charged metal cations (Mg2+, Ca2+, Zn2+, or Al3+). These enhanced electrolyte mixtures, named “MESA” (which stands for mixed-salt electrolytes for silicon anodes), give silicon anodes increased surface and bulk stabilities, improving long-term cycling and calendar life.
The Argonne researchers also investigated the functioning of the MESA-containing electrolytes. During charging, the metal cation additions in electrolyte solution migrated into the silicon-based anode, along with the lithium ions, to form lithium-metal-silicon phases, which are more stable than lithium-silicon.
Research for improved-stability lithium-ion batteries has been conducted for many decades. In 2013, researchers developed a new nonaqueous Li2B12F12-xHx electrolyte that used lithium difluoro(oxalato)borate as an electrolyte additive, which showed superior performance to conventional LiPF6-based electrolytes with regard to cycle life and safety, including tolerance to both overcharge and thermal abuse. Cells tested with the Li2B12F9H3-based electrolyte maintained about 70% initial capacity when cycled at 55 °C for 1,200 cycles. The intrinsic overcharge protection mechanism was active up to 450 overcharge abuse cycles. Results from in situ high-energy X-ray diffraction showed that the thermal decomposition of the delithiated Li1-x[Ni1/3Mn1/3Co1/3]0.9O2 cathode was delayed by about 20 °C when using the Li2B12F12-based electrolyte.
In 2016, scientists designed a battery using Li-salts of various superhalogen anions, such as BO2‾, AlH4‾, TiH5‾ and VH6‾ as well as hyperhalogen anions, BH4-y(BH4)y‾ (y = 1−4). The salts all were halogen free and hence non-toxic and safer than LiBF4, LiPF6 etc. Also, LiB4H13 and LiB5H16 were two potential candidates for electrolytic salt due to their smaller Li-dissociation energy (ΔE) than those of LiBF4, LiPF6 etc. Also, ΔE of LiBH4-y(BH4)y varied inversely with the VDE of BH4-y(BH4)y‾ anions.
In 2017, a structural battery electrolyte (SBE) as a two phase system was invented which used reaction induced phase separation. A liquid electrolyte phase was combined with a stiff vinyl ester-based thermoset matrix to form a SBE. The effect of monomer structure variations on the formed morphology and electrochemical and mechanical performance was also investigated. An ionic conductivity of 1.5 _ 10_4 S cm_1, with a corresponding storage modulus (E0) of 750 MPa, was obtained under ambient conditions. The SBEs were combined with carbon fibers to form a composite lamina and evaluated as a battery half-cell. Studies on the lamina revealed that both mechanical load transfer and ion transport were allowed between the carbon fibers and the electrolyte. These results paved the way for the preparation of structural batteries using carbon fibers as electrodes.
The advantages of this new battery design are manifold. This new cell chemistry greatly reduced the detrimental side reactions between the silicon anode and electrolyte that had occurred in cells with the traditional electrolyte. Of the four metal salts tested in cells, the added electrolyte salts with either magnesium (Mg2+) or calcium (Ca2+) cations showed the best results over hundreds of charge–discharge cycles. The energy densities obtained with these cells surpassed those for comparable cells having graphite chemistry by up to 50%.
Based on these test results scientists are positive that, if silicon anodes ever replace graphite or constitute the anode in more than a few percent concentration, this invention would be part of it and could have far reaching impact.