Are graphene lithium batteries really that cool?

Since physicists AndreGeim and KonstantinNovoselov of the university of Manchester, United Kingdom, Shared the 2010 Nobel Prize in physics for their "groundbreaking experiments with two-dimensional graphene materials," any news or research on graphene has received a lot of attention. So what are the potential advantages of graphene lithium battery recharging? For example, the popular lithium battery with mobile charge makes the audience full of expectation for the application of this technology.

I also often ask the company's research and development cattle: "graphene can now mass production? "Are graphene lithium batteries fabled to be so powerful that they can be fully charged in seconds? In fact, for a professional often can only ha ha to answer, like electricity I have no background in electrochemistry or materials science, it is difficult to understand those obscure professional theory.

The author combines professional answers and various data to sort out the following content for you to see, in today's technology changes with each passing day, it is really difficult to say those advanced ideas and ideas, now it seems like a hype, maybe that day will become a reality.

What is graphene

Graphene (add Graphene) is a two-dimensional material consisting of carbon atoms with sp2 hybridized orbitals to form a hexagonal flat film with honeycomb lattice and only one carbon atom thickness. Graphene is currently the thinest but hardest nanomaterial in the world. It is almost completely transparent and absorbs only 2.3% of light. Thermal conductivity is up to 5300W/m·K, higher than carbon nanotubes and diamond, at room temperature its electron mobility is more than 15000cm2/V·s, and higher than carbon nanotubes or silicon crystals, and the resistivity is only about 10-8 1% m, lower than copper or silver, the world's smallest resistivity of the material.

The thinnest, the hardest, the most heat-conducting, the most electrically conductive, all of these rings are telling people what an amazing material graphene is. However, the author should remind that Graphene can only be called Graphene by the international definition of 1-2 layers nanosheet, and only Graphene without any defects has these perfect properties, while the actual Graphene produced is mostly multi-layer and has defects.

Current production methods and quality

Mechanical stripping: Geim's team produced graphene by hand using 3M tape, but the yield was so low and the resulting graphene was so small that the method was not likely to be commercially viable.

Chemical vapor deposition method (CVD) : chemical vapor deposition method is mainly used to prepare graphene thin films. At high temperature, methane and other gases are deposited on the surface of metal substrate (Cu foil) for catalytic cracking and then graphene is formed. The advantage of CVD method is that it can grow graphene films with large area, high quality and good uniformity, but the disadvantage is that it is difficult to transfer due to the high cost and complex process, and the generally grown graphene films are polycrystalline.

Oxidation-reduction method: oxidation-reduction method refers to the reaction of natural graphite with strong acid and strong oxidizing substances to generate graphite oxide (GO), which is dispersed by ultrasound to prepare graphene oxide, and then added with reductant to remove oxygen-containing groups on the surface of graphite oxide to get graphene. The oxidation-reduction method has become the most mainstream method for the production of graphene due to its low cost and easy realization. However, the waste liquid generated by this method has serious environmental pollution. The prepared graphene is generally multi-layer graphene or graphene microcrystalline rather than graphene in the strict sense, and some electrical and mechanical properties of graphene are lost due to product defects.

Solvent stripping method: the principle of solvent stripping method is that a small amount of graphite is dispersed in the solvent to form a low-concentration dispersion solution. The effect of ultrasound is used to destroy the van der Waals force between the graphite layers. The solvent is inserted between the graphite layers and the layers are peeled to produce graphene. This method does not destroy the structure of graphene as the oxidation-reduction method can produce high-quality graphene. The disadvantage is that the cost is high and the production rate is very low, the industrial production is difficult.

In addition, the preparation methods of graphene include solvent thermal method, high temperature reduction, light reduction, epitaxial crystal growth method, microwave method, arc method, electrochemical method, etc., which are not as common as the four methods mentioned above.

Here, we introduce a new term: RGO. In general, go is made from graphite oxidized by a strong acid and then reduced by chemical reduction or thermal shock. At present, the vast majority of so-called "graphene" in the market are graphene oxide produced by the oxidation-reduction method. The number of graphite sheets varies, and there are a lot of defects and functional groups on the surface, whether electrical, thermal or mechanical properties are different from that of the Nobel Prize winning graphene. They are not technically "graphene".

The term "graphene battery" is hot right now. In fact, the term "graphene battery" does not exist in the international lithium battery academia and industry. So "graphene lithium battery" is a really exciting concept.

According to graphene-info, the leading Graphene website in the United States, "Graphene battery" is defined as a battery made of Graphene added to electrode materials. In my opinion, this explanation is obviously misleading. According to the classical electrochemical nomenclature, the lithium-ion battery commonly used in smart phones should be named "lithium cobalt-graphite battery".

Is referred to as the "lithium ion battery, because SONY 18650 lithium ion batteries on the market in 1991, when considering the classic nomenclature is too complex average person can't remember, and charge and discharge process is implemented by lithium ion migration, system does not contain lithium metal, therefore is called" Lithiumionbattery ". In the end, the name "lithium ion battery" was widely accepted around the world, which also reflected the special contribution of SONY in the field of lithium battery.

At present, almost all commercial lithium ion batteries are made of graphite type anode materials. In the case of similar negative electrode properties, the performance of lithium ion batteries largely depends on the anode materials. So now lithium ion batteries also have the habit of calling them by the anode. For example, BYD lithium iron phosphate battery (the so-called "iron battery" is not in the scope of the author's discussion), lithium cobalt acid battery, lithium manganese acid battery, ternary battery, etc., are all for the positive electrode.

Graphene has a possible (but only possible) application in lithium batteries

Negative:

1. Graphene is used solely for anode materials;

2. Forming composite materials with other new cathode materials, such as silicon-based and tin-based materials and transition metal compounds;

Negative conductive additive.

Positive electrode: it is mainly used as a conductive agent added to the positive electrode of lithium iron phosphate to improve the ratio and low temperature performance; There are also studies on improving the cycling performance of lithium manganese phosphate and lithium vanadium phosphate.

The actual performance of graphene functionally-coated aluminum foil is not much better than that of ordinary carbon-coated aluminum foil (developed by A123 in conjunction with hankel). On the contrary, the cost and process complexity have increased a lot, making the commercialization of this technology very unlikely.

From the above analysis, it is clear that there are only two possible areas for graphene to play a role in lithium ion batteries: directly used in cathode materials and conductive additives.

The possibility of graphene being used as a lithium anode material alone

The charge-discharge curve of pure graphene is very similar to that of hard carbon and activated carbon materials with high specific surface area, both of which have the disadvantages of very low initial cycle coulomb efficiency, too high charge-discharge platform, serious potential lag and poor cycling stability. These problems are actually the basic electrochemical characteristics of disordered carbon materials with high specific surface.

Graphene has very low vibration and compaction densities and is extremely expensive, so there is no possibility to directly use graphene as the negative electrode of lithium ion batteries instead of graphite materials. Since graphene alone is not viable as a negative electrode, what about graphene composite materials?

Graphene and other new negative materials, such as silicon-based and tin-based materials and transition metal compounds to form composites, are currently the hottest research area for "" nano-lithium" ", with thousands of papers published in the past few years. On the one hand, the flexibility of graphene sheet layer is used to buffer the volume expansion of these high-capacity electrode materials during the cycle; on the other hand, the excellent conductivity of graphene can improve the electrical contact between the particles of the material and reduce the polarization. All these factors can improve the electrochemical properties of the composite material.

However, it is not just graphene that can achieve the improvement effect. The author's practical experience shows that the application of conventional carbon composite technology and process can achieve similar or even better electrochemical properties. For example, Si/C composite cathode materials do not significantly improve the electrochemical properties of the materials compared with the common dry composite technology. On the contrary, the dispersion and compatibility of graphene increase the complexity of the process and affect the batch stability.

If the material cost, production process, processability and electrochemical properties are taken into consideration, the author believes that the possibility of actual application of graphene or graphene composites in lithium anode is very small and the prospect of industrialization is slim.

Possibility of using graphene as a conductive agent

At present, the conductive agents commonly used in lithium electricity include conductive carbon black, acetylene black, cogen black, SuperP, etc., and now some battery manufacturers begin to use carbon fiber (VGCF) and carbon nanotubes (CNT) as the conductive agents in power batteries.

The principle of graphene as a conductive agent is its excellent electronic transmission capability due to its special structure of two-dimensional high specific surface area. According to the test data accumulated so far, VGCF, CNT and graphene all have certain improvement over SuperP in terms of multiplier performance, but there is little difference in the degree of improvement of electrochemical performance among the three, and graphene does not show obvious advantage.

So could adding graphene make the electrode material explode? The answer is no. IPhone mobile phone batteries, for example, the battery capacity of ascension is largely due to the result of the ascension of the LCO working voltage, maximum charging voltage from 4.2 V to 4.35 V, currently on the I - Phone6 makes LCO capacity from 145 mah/g gradually increase to 160-170 mah/g (high pressure LCO must pass a doping and modification measures such as surface coating), all of these improve has nothing to do with graphene.

In other words, if you use lithium cobalt oxide with a cut-off voltage of 4.35v and a capacity of 170mAh/g at high pressure, you cannot increase the capacity of lithium cobalt oxide to 180mAh/g by as much as you add graphene, not to mention the so-called "graphene battery" with several times the capacity. Is adding graphene likely to improve battery life? It's also impossible. The specific surface area of graphene is larger than that of CNT, and the addition of CNT to the negative electrode can only generate more SEI and consume lithium ions, so CNT and graphene can only be added to the positive electrode to improve the ratio and low temperature performance.

However, the rich functional groups on the graphene surface are the small wounds on the graphene surface. Excessive addition will not only reduce the energy density of the battery, but also increase the amount of liquid absorbed by the electrolyte. On the other hand, it will increase the side reactions with the electrolyte and affect the circulability, and may even cause safety problems. What about the cost? Graphene is currently extremely expensive to produce, and the so-called cheap "graphene" products on the market are basically graphene oxide.

Even go costs more than CNT, which is higher than VGCF. And in terms of dispersibility and processability, VGCF is easier to operate than CNT and graphene, which is the main reason why showa denko's VGCF is gradually entering the power battery market. It can be seen that graphene does not have any advantage over CNT and VGCF in cost performance when used as conductive additive.

The current hot situation of graphene in China reminds me of carbon nanotubes (CNTS) more than ten years ago. If we compare graphene and CNT, we will find that the two are strikingly similar, with many almost identical "strange properties". These "magic properties" of CNT then are now completely applied to graphene. CNT began to catch fire internationally at the end of the last century and reached its climax between 2000 and 2005. CNTS are said to be very versatile and have many "unique properties" in the lithium field.

But more than 20 years have passed, and so far have not seen these CNT "strange performance" in any field has a real large-scale application. In terms of lithium electricity, CNT is only used as a positive electrode conductive agent. In the past two years, it has started a small-scale trial in LFP power battery (the cost performance is still lower than that of VGCF), and LFP power battery is doomed to become the mainstream technology route of electric vehicles.

Compared with CNT, graphene is very similar to it in terms of electrochemical properties without any special features. On the contrary, it has higher production cost, more serious environmental pollution in the production process and more difficult practical operation and processing performance. Based on my years of experience in lithium battery development and production, I don't think graphene will have much practical application value in the field of lithium ion batteries. The so-called "graphene battery" is just hype. Comparing CNT with graphene, the author would like to say that "history is always so similar".

The real potential applications of graphene have been speculated

Future applications of graphene in lithium ion batteries are very limited. Compared with lithium ion batteries, the author thinks that the application prospect of graphene in supercapacitors, especially in the field of micro supercapacitors, seems to be a little bit more reliable. However, we should still be alert to some academic hype.

In fact, after reading many of these so-called "academic breakthroughs", you will find that many professors have intentionally or unintentionally confused some basic concepts in their paper. Currently, commercial activated carbon supercapacitors generally have an energy density of 7-8wh /kg, which refers to the device energy density of the entire supercapacitor containing all the components. And the breakthrough that professors talk about is usually the energy density of the material, so the actual graphene supercharge is not nearly as good as the paper suggests.

Relatively, the cost of miniature super capacitor requirements is not as strict, capacitors with graphene composites as electrochemical active material, and choose the appropriate ionic liquid electrolyte, the preparation can be realized with double advantage on traditional capacitor and lithium ion battery energy storage devices, in microelectromechanical system (MEMS) such a small niche areas might (just) may have certain application value.

The page contains the contents of the machine translation.