Research on safer, more economical, and higher energy density aqueous
lithium-ion batteries has ushered in an important breakthrough. In the latest
research progress, the team of Robert Franklin and Frances Riggs Wright
Distinguished Chair Professor Wang Chunsheng of the Department of Chemical and
Biomolecular Engineering at the University of Maryland Dr. Yang Zhongyin and Dr.
Chen Ji used halogen elements (bromine, chlorine) + graphite to develop a
cathode material that is completely different from traditional lithium
batteries, creating an aqueous lithium-ion battery with the highest energy
density of 304Wh/kg. At the same time, the battery It has obvious advantages
over currently common lithium-ion batteries in terms of safety and economy.
The energy density of this cathode material is about 1.5-2 times that of
today’s commercial lithium cobalt oxide cathodes. Paired with a graphite
negative electrode and a high-safety aqueous electrolyte, the battery energy
density reaches a maximum of 304Wh/kg (including electrolyte), which is
equivalent to or even higher than the energy density of current commercial
lithium-ion batteries. The results were recently published in Nature. The first
authors of the paper are Yang Chongyin and Chen Ji, researchers in the
Department of Chemical and Biomolecular Engineering at the University of
Maryland.
At present, the team has used this technology to create small button
batteries in the laboratory and has reached a cooperation with French battery
manufacturer Saft, which is expected to be commercialized in the near
future.
Safe and economical aqueous lithium-ion battery
Battery safety is related to the personal and property safety of consumers,
and has always been a focus issue. A common symptom of unsafe batteries is
thermal runaway. Under internal short circuit, high-current charge/discharge,
overcharge, etc., a large amount of heat is generated inside the battery. When
it reaches a higher temperature, there is a risk of burning or explosion.
Today, the more mature and widely used lithium-ion batteries are organic
lithium-ion batteries, that is, the electrolytes in the batteries are highly
flammable organic solutions. This causes the solution to easily catch fire or
even explode in the event of thermal runaway. At the same time, increasing the
energy density of the battery will further increase the possibility of thermal
runaway to a certain extent and reduce the safety of the battery. This is also
the bottleneck of the development of lithium batteries.
The concept of aqueous lithium-ion batteries was first proposed in 1994 by
the famous Canadian lithium battery scientist J. R. Dahn. The biggest feature of
aqueous lithium-ion batteries is that the electrolyte of the battery is not an
organic solution but an aqueous solution. Since the aqueous solution is not
flammable, it is even highly flame retardant. sex and therefore significantly
safer. In terms of performance, the conductivity of aqueous lithium-ion
batteries is 1-2 orders of magnitude higher than that of organic systems, so the
power is better; in addition, aqueous lithium-ion batteries are lower in cost
and less polluting.
But at the same time, the problem of aqueous lithium-ion batteries is also
very prominent. In previous studies, due to the low decomposition voltage of
pure water (1.23V), the stable operating voltage of previous aqueous lithium-ion
batteries could not even exceed 2V, but we The working voltage of batteries in
daily use is usually 3-4V. Therefore, aqueous lithium batteries are still unable
to meet the energy density requirements for daily use. This is also the key
reason why traditional lithium batteries cannot get rid of organic
electrolytes.
As early as 2015, Wang Chunsheng's team collaborated with the U.S. Army
Laboratory to propose a "water-in-salt" high-voltage window aqueous electrolyte
(WiSE for short). This electrolyte can effectively reduce the activity of water
and generate electricity at the negative electrode during operation. A
protective solid barrier is formed around it to prevent water from being
electrolyzed into hydrogen and oxygen. This research increases the redox
potential window of aqueous solutions in batteries to around 3 V. This result
means that aqueous lithium-ion battery research has broken through the key
voltage limit. The results were published in the journal Science. By 2017, Wang
Chunsheng’s team invented a new negative electrode protection strategy, which
expanded the original aqueous electrolyte window to more than 4 V.
The next research work is to find positive and negative electrode materials
that match WiSE to further increase the energy density. In the latest research,
the team developed a cathode material that is completely different from
traditional lithium batteries and matched with a highly safe aqueous
electrolyte. This key result was recently published in the journal Nature.
So far, Wang Chunsheng's team has achieved breakthroughs in electrolytes,
positive electrodes, and negative electrode materials, and has assembled a
high-voltage aqueous lithium-ion battery system with an operating voltage above
4V.
Specifically, this new cathode material breaks through the inherent idea of
the original cathode material relying on transition metal elements, and uses
bromine and chlorine elements to undertake the redox process. The high
concentration of lithium salt in the electrolyte can effectively prevent bromine
and chloride ions from moving around, locking them in the solid salt particles
around the electrode, preventing them from being affected by the aqueous
electrolyte.
When charging the battery, the hydrated bromide ions and chloride ions in
the positive electrode undergo an oxidation reaction, releasing electrons,
oxidizing them into bromine and chlorine atoms, and embedding them between the
carbon layers of the graphite positive electrode to form a solid layer. On the
other side, the positively charged lithium ions reach the negative electrode
through the aqueous electrolyte, undergo a reduction reaction with the electrons
arriving through the external current, and are embedded between the carbon
layers of the graphite negative electrode, and charging is completed.
Later, during the battery discharge process, the metallic lithium between
the carbon layers of the graphite negative electrode releases electrons and
turns into lithium ions. At the same time, electrons also pass through the
external circuit from the negative electrode to the positive electrode during
the discharge process. Bromine and chlorine atoms receive electrons and become
bromide ions and chlorine ions respectively. At this time, the water-in-salt
electrolyte blocks the movement of flowing bromine and chloride ions,
regenerating solid salt particles in the positive electrode until the next round
of charging.
Cathode material: a key link in determining the performance of lithium
batteries
A common lithium battery generally includes positive electrode material,
negative electrode material, electrolyte, separator and battery casing. The
positive electrode, negative electrode, and electrolyte are all critical parts
of a battery.
The cathode material is a key link in limiting the energy density of
lithium-ion batteries. Simply put, as a whole system, the energy density of the
battery is related to the positive electrode, negative electrode, and
electrolyte, and the positive electrode material is the shortcoming of the
three. The capacity of graphite electrodes commonly used as negative electrodes
has reached 350Wh/kg a long time ago. But for cathode materials, even the
current cutting-edge NCM811 has a capacity of only about 200Wh/kg.
For lithium-ion batteries, it is common practice to use transition metal
oxides as cathode materials. Yang Chongyin said that this kind of transition
metal oxide usually has a layered structure or a porous structure, which can
ensure that the structure does not collapse during the process of allowing
lithium ions to freely enter and exit. But the problem is that the metal oxide
skeleton itself is very heavy, and this part itself cannot provide capacity,
which reduces the energy density of the battery in disguise.
The second type is the positive electrode that relies on the conversion
reaction of sulfur, oxygen and other anions to change their price (such as
lithium-sulfur batteries, sulfur and air electrodes in lithium-air batteries).
Since their material weight is very small, the theoretical capacity they can
obtain is very high. However, the disadvantage is that it lacks a fixed
structural skeleton, so the reversibility is relatively poor and the cycle life
of the battery cannot be guaranteed.
The team proposed using halogen elements (including bromine and chlorine)
in the cathode material, which have a higher potential than oxygen elements. But
one of the problems with doing this is that elements such as bromine and
chlorine form liquid and gaseous elements after oxidation, which brings great
trouble to the direct use of batteries. In order to solve this problem, the team
adopted a new idea, which is to use graphite to fix the oxidized active
materials.
This method combines the advantages of two traditional cathode materials.
It not only takes advantage of the high capacity and high voltage of halogen
anion redox, but also takes advantage of the easy reversible
insertion/extraction of halogen elements between graphite layers to improve
stability.
Experiments have shown that the theoretical capacity of this cathode
material can be higher than that of traditional transition metal oxides. The
paper shows that the cathode energy density of the battery reaches 970Wh/kg,
which is almost twice that of the commercial transition metal intercalated
cathode lithium cobalt oxide. After counting the total mass of the positive
electrode, negative electrode and electrolyte, the energy density of the aqueous
full battery is approximately 304Wh/kg, which is equivalent to or even higher
than the energy density of current commercial lithium-ion batteries.
Of course, this technology is not yet ready for use. Chen Ji told DeepTech
that this research is still in the early proof-of-concept stage, and the team
has now used this technology to make smaller button batteries in the laboratory.
There is still a lot of follow-up work that needs to be done before it can be
put into actual use. For example, the stability of graphite materials in aqueous
solutions needs to be further improved to meet the commercial requirements of
batteries, and these require further verification and optimization.
At the same time, the work of putting the technology into industrialization
has begun. Yang Chongyin said that he is currently cooperating with French
battery manufacturer Saft. The team is responsible for providing technology, and
Saft is responsible for amplifying the battery technology and creating a larger
battery prototype.
In addition, in addition to improving the performance of aqueous
lithium-ion batteries, Yang Chongyin is also exploring the possibility of
applying this cathode material to solid-state batteries and non-aqueous
lithium-ion batteries.
Future directions for battery research
With the obvious electrification trend represented by the automobile
industry in recent years, battery research has received considerable attention.
At present, in addition to traditional lithium batteries, solid-state batteries
and aqueous lithium-ion batteries have also set off a research boom. Among them,
high energy density and high safety are two key factors in battery research. How
to increase energy density while ensuring safety is a common pursuit of academia
and industry.
Regarding the future development of lithium batteries, Wang Chunsheng
believes that the current organic system has developed to an obvious bottleneck
stage, that is, when increasing energy density, it is difficult to ensure good
safety. Therefore, increasing energy density without losing safety is an
inevitable direction for battery research. This is the potential of solid-state
batteries and aqueous lithium-ion batteries.
Wang Chunsheng believes that both directions have great prospects and
hopes, and at the same time, they also have their own problems that need to be
solved. He said that the current organic system batteries are very mature in
terms of stability, which is also a technical problem that aqueous lithium-ion
batteries and solid-state batteries need to overcome.
At this stage, it is difficult to determine the advantages and
disadvantages between solid-state batteries and aqueous lithium-ion batteries,
but it is certain that as long as there is a breakthrough, it will definitely
become a better choice than traditional lithium-ion batteries in some usage
scenarios. Considering that these two technical routes have attracted a large
number of researchers and research funds, I believe that key breakthroughs will
come soon.
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