It is difficult to imagine our daily life without lithium-ion batteries.
They dominate the small format battery market for portable electronic
devices, and are also commonly used in electric vehicles. At the same time,
lithium-ion batteries have a number of serious issues, including: a
potential fire hazard and performance loss at cold temperatures; as well as
a considerable environmental impact of spent battery disposal.
According to the leader of the team of researchers, Professor in the
Department of Electrochemistry at St Petersburg University Oleg Levin, the
chemists have been exploring redox-active nitroxyl-containing polymers as
materials for electrochemical energy storage. These polymers are
characterised by a high energy density and fast charging and discharging
speed due to fast redox kinetics. One challenge towards the implementation
of such a technology is the insufficient electrical conductivity. This
impedes the charge collection even with highly conductive additives, such as
carbon.
Looking for solutions to overcome this problem, the researchers from St
Petersburg University synthesised a polymer based on the nickel-salen
complex (NiSalen). The molecules of this metallopolymer act as a molecular
wire to which energy-intensive nitroxyl pendants are attached. The molecular
architecture of the material enables high capacitance performance to be
achieved over a wide temperature range.
'We came up with the concept of this material in 2016. At that time, we
began to develop a fundamental project "Electrode materials for lithium-ion
batteries based on organometallic polymers". It was supported by a grant
from the Russian Science Foundation. When studying the charge transport
mechanism in this class of compounds, we discovered that there are two keys
directions of development. Firstly, these compounds can be used as a
protective layer to cover the main conductor cable of the battery, which
would be otherwise made of traditional lithium-ion battery materials. And
secondly, they can be used as an active component of electrochemical energy
storage materials,' explains Oleg Levin.
The polymer took over three years to develop. In the first year, the
scientists tested the concept of the new material: they combined individual
components to simulate the electrically conducting backbone and redox-active
nitroxyl-containing pendants. It was essential to make certain that all
parts of the structure worked in conjunction and reinforced each other. The
next stage was the chemical synthesis of the compound. It was the most
challenging part of the project. This is because some of the components are
extremely sensitive and even the slightest error of a scientist may cause
degradation of the samples.
Of the several polymer specimens obtained, only one was found to be
sufficiently stable and efficient. The main chain of the new compound is
formed by complexes of nickel with salen ligands. A stable free radical,
capable of rapid oxidation and reduction (charge and discharge), has been
linked to the main chain via covalent bonds.
'A battery manufactured using our polymer will charge in seconds - about ten
times faster than a traditional lithium-ion battery. This has already been
demonstrated through a series of experiments. However, at this stage, it is
still lagging behind in terms of capacity - 30 to 40% lower than in
lithium-ion batteries. We are currently working to improve this indicator
while maintaining the charge-discharge rate,' says Oleg Levin.
The cathode for the new battery has been fabricated - a positive electrode
for use in chemical current sources. Now we need the negative electrode -
the anode. In fact, it does not have to be created from scratch - it can be
selected from the existing ones. Paired together they will form a system
that, in some areas, may soon supersede lithium-ion batteries.
'The new battery is capable of operating at low temperatures and will be an
excellent option where fast charging is crucial. It is safe to use - there
is nothing that may pose a combustion hazard, unlike the cobalt-based
batteries that are widespread today. It also contains significantly less
metals that can cause environmental harm. Nickel is present in our polymer
in a small amount, but there is much less of it than in lithium-ion
batteries,' says Oleg Levin.
Reference:
“The Fast and the Capacious: A [Ni(Salen)]‐TEMPO Redox‐Conducting Polymer
for Organic Batteries” by Anatoliy A. Vereshchagin, Dr. Daniil A. Lukyanov,
Ilia R. Kulikov, Dr. Naitik A. Panjwani, Dr. Elena A. Alekseeva, Prof. Jan
Behrends and Prof. Oleg V. Levin, 17 November 2020, Batteries and Supercaps.