What is the value of Graphite batteries from biochar?

Combating global climate change will require vast utilization of bioenergy and carbon negative or neutral “green” materials production and utilization1. It has been proposed that these processes should be made economically competitive by carbon valuation because finding market competitive solutions to replace carbon pricing (e.g. carbon tax or emission permits) is extremely challenging. Lignocellulose pyrolysis is a particularly attractive biomass conversion process, providing sustainable and carbon–neutral electricity, liquid biofuel and chemical feedstocks2. A growing number of large scale biomass pyrolysis plants have come online in recent years including a full scale combined heat and power plant (district heat, 210 GWh electricity, and 50,000 ton/year bio-oil) in Joensuu, Finland. Biomass pyrolysis oil is the least expensive carbon neutral replacement for transportation fuels, which account for 27% of all greenhouse emissions, but even so, it is not market competitive with fossil fuels. Attaining market viability depends on driving down the cost of bio-oil by upgrading or valorization of its generally burned waste product, biochar, to value-added products3.

Graphite is an attractive target valorization product of biochar. Graphite is classified as a “strategic and critical mineral” by the US and EU, with a market that is expected to nearly double to reach $28.33 billion by 20264. While graphite is used in numerous applications, its market growth is expected to be driven by the increasing demand for Li-ion grade graphite, with Li-ion battery “mega-factories” being built to supply the needs of electric vehicles (EV). Meeting the graphite needs to limit global temperature rise to 2 °C will require a 500% increase in production by 2050 according to a recently published report from The World Bank5. However, severe supply shortages are predicted6,7, with potentially negative impacts to EV production and increased costs. Thus, conversion of biochar to Li-ion grade graphite could help meet the needs of a very large and growing market, while increasing its value by ~ 1000 fold8, and enabling the material needs for the wide adoption of zero emission EV’s.

Current graphite production, whether obtained through high temperature (3000 °C) transformation (synthetic graphite) of highly pure graphitizable carbons or mining (natural graphite), is highly deleterious to the environment. Synthetic graphite production is highly energy intensive (~ 7500 kWh/t)9 and results in large greenhouse gas emissions10. Mining is devastating to the landscape and purification of natural graphite requires large-scale use of environmentally harmful agents such as HF and H2SO411. Micronizing and shaping (“rounding”) the graphite, necessary for Li-ion battery application, results in significant (~ 70%) material loss. Coating and “re-graphitizing” adds further environmental impact and cost. While the academic researchers actively participated in the development and improvement of graphite anodes for Li-ion batteries, their contributions have dwindled in the past decade due to the complexity and facility requirements of conventional graphitizing and processing to produce graphite appropriate for Li-ion batteries, limiting research primarily to modifications, applications and properties of systems based on commercial graphite.

Graphite is entrenched as the predominant anode active material in commercial Li-ion batteries, and is likely to remain so for the foreseeable future despite intense research efforts over the past two decades to find a higher energy density replacement. Most recently, Si has been of particular interest as a replacement for graphite as an anode material, due to its extremely high gravimetric and volumetric capacities, almost ten times higher than those of graphite, low working potential and abundance. However, development has been challenging due to the very large expansion of Si upon lithiation, up to ~ 300%, which leads to electrode structural degradation during cycling, ineffective passivation and low Coulombic efficiency (CE). Even so, electric vehicle (EV) battery manufacturers have successfully incorporated small percentages Si materials, generally SiOx, as capacity boosting additives to graphite anodes in commercial cells. It has been proposed that practical incorporation of Si into Li-ion cells requires a similar strategy, that is, being blended into composites with graphite12. While numerous investigations of composites of Si and Li-ion graphite have been reported recently, they have employed commercial Li-ion graphites that were tailored by industry to achieve excellent performance as the sole or primary active material, including high packing density and thus volumetric capacity, rather than optimal properties, including shape and porosity, to host Si13,14,15,16,17,18,19,20,21,22. As was pointed out recently, development of Si/graphite composites requires that “both the graphite and Si parts should be engineered”12, a task that is greatly encumbered by the need to use commercial graphite as a starting material for modification and blending, rather than synthesizing customized graphite.

Recently, it was first reported that biochar and other non-graphitizable carbons could be converted to high purity (99.95% C), highly crystalline graphite (biochar graphite or BCG) at a laboratory scale with equipment requirements readily within the financial means of many academic laboratories8. It was shown that biomass could be readily converted to “potato” graphite, morphologically similar to some commercial graphites, however, its relatively high surface area (10.3 m2/g) resulted in low initial CE, 84%, unacceptably below that of commercial Li-ion anode graphite (~ 90%). While the control of graphite flake size, a critical parameter for Li-ion battery performance, was demonstrated, the ability to rationally select the agglomerate shape and size was not, nor was an understanding of the origin of the morphology obtained presented. Additional reports have appeared demonstrating the conversion of biomass to anode materials with varying degrees of graphitic character and battery performance, but without rational morphology control23,24,25,26,27,28.

In an effort to gain mechanistic insight into the BCG graphitization process, the effects of process parameters on yield and agglomerate morphology were studied and are presented herein. It is shown that the size and shape of the graphite agglomerates can be rationally controlled to a remarkable degree. Finally, the rational synthesis of graphite with a predetermined flake size, and agglomerate size and shape, is demonstrated and shown to closely compare to commercial Li-ion graphite. The methods described will allow researchers to produce graphite at a laboratory scale with tailored morphological properties, including porosity, to tailor its performance as anode active material and/or alloying material host, with the potential to scale to industrial quantities, without the environmental impact of current graphite production.