Graphene Batteries: A Myth Or A Bubble?

Challenges facing lithium-ion batteries

Over the past two decades since the advent of lithium-ion batteries, our world and life have brought earth-shaking changes. The high specific energy and high power working requirements of energy storage devices such as consumer electronic equipment and electric vehicles have made the existing lithium-ion batteries "stressed". The innovation of battery technology has fallen far behind the upgrading of electronic equipment and has become a constraint on user experience. The biggest bottleneck.

Traditional lithium-ion batteries are based on the shuttle of active lithium ions between the positive and negative materials to achieve the conversion of chemical energy and electrical energy. However, it is precisely this electrochemical mechanism of insertion and extraction that makes the capacity and energy density of lithium-ion batteries increasingly unable to meet the needs of application scenarios. In terms of negative electrode materials, the negative electrode materials of commercial lithium-ion batteries represented by graphite use lithium ions to deintercalate between graphite layers to work. However, the sites of lithium in graphite and the interlayer spacing of graphite itself are very limited, which forces lithium-ion batteries to face the dilemma of insufficient capacity and low specific energy.

Graphene battery: turned out

At a time when people are at a loss, a new type of star carbon material-graphene has come out! Graphene can be regarded as a single-layer graphite, which has abundant lithium intercalation sites, and has ultra-high electronic conductivity and a huge specific surface area. In this way, can graphene replace graphite to detonate a revolution in the energy storage industry? With high capacity, high energy density, and fast charging, don't these "Peach Blossom Springs" that people have been pursuing directly become reality? ! Various media have also started to report on the advantages of graphene batteries and make corresponding hype. For a time, graphene battery-related concept stocks have become popular. The entire battery industry seems to have been beaten. Everyone is looking forward to graphene batteries. The arrival of the times.

However, is this really the case? The following content is mainly from a scientific point of view to uncover the veil of the mysterious graphene battery for everyone (Note: Graphene battery has not yet a clear concept, according to the role of graphene can be roughly divided into graphene as a conductive additive and graphite There are two types of ene as the negative electrode material. This article discusses graphene as the negative electrode material of the battery).

origin

In 2014, Scientific Report reported a work on all-graphene lithium batteries. In this all-graphene battery, the positive electrode is surface-functionalized graphene material, and the negative electrode is reduced graphene oxide. The entire battery utilizes the surface reaction of the positive and negative electrodes, so it can achieve super-high rate charge and discharge. The power density calculated based on the overall electrode mass can reach 2150W/kg.

From the power density point of view, the battery is indeed promising, but when we look at the energy density again, we can find that the energy density calculated based on the mass of the two electrodes is only 130Wh/kg, which is just able to reach the existing lithium-ion battery based on the system mass calculation (The system energy density of the recently popular BYD blade battery is 140Wh/kg; "Made in China 2025" clearly proposes that the single energy density of vehicle-mounted power batteries should reach 300Wh/kg by 2020). If it is integrated into a battery system, its mass energy density will be discounted by another five to sixty percent. Moreover, the positive and negative electrode materials of this all-graphene battery do not contain lithium, so electrochemical pre-lithiation in the half-cell must be carried out before matching into a full battery. Looking at it this way, graphene batteries may be the first to develop in high-power scenarios, but their energy density is still far from people's expectations.

So in theory, can graphene be used as a negative electrode material for batteries like graphite? Is the mechanism of lithium insertion the same as graphite? What is its theoretical lithium storage capacity? Many researchers believe that because graphene has two sides that can adsorb lithium atoms, it can form a dual lithium phase of Li2C6 and has a double specific capacity of 744 mAh/g. There are many researches on these issues. Some researchers have used DFT calculations to find that lithium atoms cannot be directly adsorbed on the surface of graphene. They can only be embedded between graphene layers or into the middle of graphene and substrate through edges or high-order defects. So in this case, is it deintercalation or adsorption, and how many Li atoms can be stored?

Shattered

In response to this problem, Associate Professor Ji Kemeng of Tianjin University reported his research on the lithium intercalation mechanism of double-layer graphene in Nature Communications in 2019. They used a high-temperature switching chemical vapor deposition method to prepare a double-layer graphene material with a high specific surface. This material does not need to be attached to the substrate and has few defects, so the influence of the substrate and defects on the adsorption or deintercalation of lithium ions can be eliminated, which is beneficial to the study of the mechanism of deintercalation of lithium in graphene itself. Constant current charge-discharge tests and cyclic voltammetry curves show that double-layer graphene has the same electrochemical oxidation-reduction reaction as conventional graphite electrodes, and lithium ions are deintercalated between the two graphene sheets. The graphene layer spacing is the only space for lithium storage, and the idea of absorbing and storing lithium is self-defeating! There is also a noteworthy phenomenon. The maximum capacity of double-layer graphene is only 180 mAh/g in the current density range of 0.2-50 A/g. The subsequent phase characterization shows that the stoichiometric composition of the lithium storage phase is LiC12 and The LiC6 of the non-graphite electrode is not the so-called dual lithium storage Li2C6 phase.

This research result shows that Daumas-Hérold's domain model is more suitable for describing the lithium storage behavior of graphite electrodes than Rüdorff's model, and has ended the half-century debate on the lithium storage mechanism of graphite. At the same time, the theoretical lithium storage capacity of graphene has finally been confirmed, and the theoretical capacity of 180mAh/g is far inferior to the electrochemical lithium storage capacity of the graphite anode. The graphene battery bubble bursts itself!

Traceability

So, where does the high capacity of graphene reported in many documents come from? We know that the graphene materials that people usually make are not relatively pure graphene like the above. Many of the graphenes we can get are rich in defects (including both the intrinsic vacancy defects of carbon materials and the defects caused by specially introduced heteroatom sites), and the surface is rich in a variety of functional groups (such as carboxyl, hydroxyl, These groups are easy to chemically interact with lithium, such as epoxy groups). The superposition of these factors and the huge specific surface area of graphene itself will cause a large amount of lithium to not participate in the electrochemical reaction in the form of deintercalation, but to contribute to the pseudocapacitance in the form of adsorption. These pseudocapacitance effects make it appear that the graphene capacity is very high and the electrochemical kinetics is fast, but this has little effect on the increase of the energy density of the full battery. Moreover, the abundant reaction sites and high defect content will also cause the limited active lithium to be continuously consumed, resulting in a decrease in the coulombic efficiency, which is fatal to the capacity stability of the full battery.

future

After the above analysis, graphene as a negative electrode material for batteries is hopeless if it wants to enter thousands of households. However, this does not mean that graphene is useless in the field of energy storage. In addition to lithium storage performance, graphene itself also has ultra-high electrical conductivity and excellent thermal conductivity. The two factors of electricity and heat play a pivotal role in actual batteries. Especially heat, battery safety accidents induced by thermal runaway can even veto many electrode materials with excellent electrochemical performance. If the advantages of both electrical and thermal conductivity are applied to the battery, the "graphene battery" may also shine.

Of course, as a kind of magical material, graphene does not know whether it will bring a new revolution to the battery in other ways? Just like the recent media reports from unknown sources, Mercedes-Benz is developing a graphene-based organic battery. The specific technology has not yet been disclosed. Anyway, it will be at least 10 years later. Whether it is a new revolution or a new bubble, we will wait and see!

In short, the field of energy storage, which aims at practicality, is not "chasing stars". The theoretically feasible graphene negative electrode requires too harsh conditions (perfect graphene). In actual production, it is necessary to pay a high cost price, which is contrary to the original intention of increasing energy density and reducing production costs. What's more, the theoretical feasibility has finally proved to be not feasible. Next time there will be media hype about "graphene battery", you have to keep your eyes open to see clearly