Although lithium ion batteries are considered as system of choice for variety of mobile and stationary applications, fundamental knowledge is alarmingly required to uncover the underlying principles controlling the basic processes that determine and dictate their function, operation, performance limitations as well as failure."

Source : Proeceedings of the International Meeting on Lithium Batteries, 2016

Because of their crucial importance as the current battery-of-choice for electric vehicles and portable electronic devices, lithium-ion batteries (and lithium-ion polymer batteries - which use a 'dry' electrolyte) are the focus of intense research. In 2021 alone they were the subject of more than 5,000 published research papers.

Although much of the electro-chemistry has now been extensively described (see Wikipedia) many fundamental aspects of the way they function are not yet fully understood.

A 2017 paper in Nature, highlights new work from the Center for Molecular Spectroscopy and Dynamics, in Korea where researchers have succeeded in observing, in realtime, the ultrafast dynamics of lithium ions with femtosecond time resolution - however :

Despite various experimental evidences indicating that lithium ion forms a rigid and stable solvation sheath through electrostatic interactions with polar carbonates, both the lithium solvation structure and more importantly fluctuation dynamics and functional role of carbonate solvent molecules have not been fully elucidated yet with femtosecond vibrational spectroscopic methods."

Possible candidates for new materials for the anodes and cathodes of the batteries are constantly being discovered - mostly by trial-and-error. Examples, too numerous to list, include : phosphorus, rock salt, nano-silicon with seaweed extract, and even carbonized garlic skin . Some of the new materials significantly outperform the currently-used technologies - but the technical reasons for the improvements are for the most part, completely unexplained.

The solid electrolyte interphase (SEI) is a nanoscale film which forms from electrolyte decomposition at the anodes of lithium-ion batteries (LIBs) during initial charging. It's been found to be a critical component for the operation modern rechargeable LIB electrolytes. Much about how the layer forms and operates is unknown.

In spite of the SEI’s central importance to battery performance and lifespan, much remains unknown regarding the formation mechanisms of the SEI in LIBs. The SEI is formed as a result of numerous competitive reactive processes occurring simultaneously over time scales ranging from picoseconds to days."

Source : Lawrence Berkeley National Laboratory , ChemRvix 2021

Complex chemical compounds known as electrolyte additives are routinely used to improve the batteries' performance - in terms of charge times, charge retention, durability etc.

Although electrolyte additives have been extensively used in modern Li-ion batteries, the practice remained a 'dark-art” with little rationale understanding."

Source : Energy Storage Materials , Volume 20, July 2019, Pages 208-215