Investigating lithium-ion battery cathode materials for new generation improvements in sustainable energy solutions
“Ensuring access to affordable, reliable, sustainable and modern energy for all will open a new world of opportunities for billions of people through new economic opportunities and jobs, empowered women, children and youth, better education and health, more sustainable, equitable and inclusive communities, and greater protections from, and resilience to, climate change.”
UN Sustainable Development Goal 7 – Energy[1].
Lithium-ion batteries are used around the world in everyday portable electronics, in electric vehicles as well as in small power grids. Scientists from the Energy Materials Research Group at the University of the Witwatersrand (Wits), South Africa, study the materials needed to improve the performance, safety, affordability and environmental footprint of lithium-ion batteries in line with important sustainable development goals (SDGs). A range of different cathode materials, as well as battery chemistry is studied to increase the understanding of the materials themselves: how various synthetic routes introduce impurity phases in these materials and how this can be avoided, and what the effects are of intentional doping and co-doping of these materials[2] to explore the impact they have on the structure, performance and impurities formed.
In this work, however, laboratory-based measurements do not often reveal a clear picture of the overall structure of the material, particularly when it contains lower concentrations of impurity phases which are either below the limit at which it can be detected or cannot be resolved from the major phase. It is therefore important to firstly, employ multiple techniques to provide complementary information and secondly, to obtain high-resolution measurements from synchrotron-based techniques which expose far more in terms of other phases in the samples. This is where the GCRF START grant plays a vital part.
“A synchrotron is millions of times more capable than the equipment in our labs in terms of brightness and detail, which makes access through the GCRF START grant to the UK’s national synchrotron, Diamond Light Source (Diamond), so significant.”
GCRF START Co-I, Prof. David Billing[3], Professor in the School of Chemistry and Co-PI of the Energy Materials Research Group at Wits.
“Thus far we have data from various synchrotrons for high resolution X-ray diffraction and total scattering, as well as X-ray absorption spectrometry,” says Group Co-PI, Prof. Caren Billing, lecturer and Associate Professor in the School of Chemistry. “The GCRF START grant provides us with these important experimental opportunities, alongside vital skills training and knowledge exchange for building capacity and training emerging scientists in the energy materials field.”
The ultimate aim of the Group is to address global energy, climate, and health challenges which, amongst other aims, includes enabling better “access to clean and safe cooking fuels and technologies and expanding the use of renewable energy beyond the electricity sector, as well as to increase electrification in sub-Saharan Africa”[4]. To this end, improved battery storage solutions are one of a creative mix of options the Group is examining, with the help of their collaborators, including the GCRF START grant.
“GCRF START asks us to consider questions in our research around the global challenges like: am I using components that are sustainable? Are we using elements which are abundant, affordable, and environmentally compatible?”, explains Prof. Dave Billing. “In doing so, we are mindful that the economic and social situations in Europe are different to Africa: different resources, different engineers, and different environments where the solutions have to work. Everything costs energy, fundamentally, and the whole solution has to fit the community – these are the bigger picture questions that START encourages us to ask.”
Some of the Group’s early career scientists focus on various lithium metal ion phosphate materials, where they are studied with the idea of using earth abundant metal ions of benefit for cost effective production and materials with a possible impact on local mining opportunities[5] to support local economies. Two of these scientists – Michelle Thiebaut from South Africa and Michelle Nyoni from Zimbabwe – are PhD research students at Wits working on lithium iron phosphate and lithium vanadium phosphate respectively, examining them as cathode materials. Both students are supervised and mentored by Prof. Dave Billing and Prof. Caren Billing, and are part of a growing number of female scientists in the Energy Materials Research Group at Wits. Michelle Thiebaut and Michelle Nyoni describe the aims, techniques and motivation for their projects in the case studies below.
Michelle Thiebaut’s research: studying Lithium iron phosphate for battery cathode materials
“When Michelle Thiebaut first started in the group, she referred to herself as “an analogue girl in a digital world”. Michelle is now the forerunner in the group in processing XAS data and has been the only chemistry student to operate the Mössbauer spectroscopy instrument in the School of Physics at Wits.” – Prof. Caren Billing, Prof. Caren Billing, University of the Witwatersrand.
Being a new researcher in a field such as energy materials is both daunting and exciting because this field is always changing and improving. One needs to change and improve one’s skill set just as quickly but the key to continue is finding one’s motivation. My main motivation is seeing how people in South Africa and in other developing countries are struggling with everyday tasks, especially people in the rural areas – tasks like coming home and doing their homework. These tasks are things many people take for granted but I think it is unacceptable that people should be struggling to get by without proper, cheap and a long-lasting access to clean energy and electricity.
My second motivation is our planet. Every person has an obligation to the planet and to live an environmentally cleaner life. By pushing science in the energy materials field means also pushing towards a greener tomorrow. Trying to break through in this field as a young female is still a bit tough with people questioning your skill set and abilities but I do think we owe former female scientists a great deal of respect for paving the way for us.
My research field is energy materials, specifically investigating the cathode material LiFePO4 found in lithium-ion batteries. My focus with this material is to find a low cost, low energy synthetic route and to possibly improve the performance. LiFePO4 is a naturally occurring mineral but can also be synthesised in a lab. This naturally occurring mineral is not phase pure[6], meaning that the iron is commonly mixed with other metals such as manganese, magnesium and calcium which lowers the electrochemical performance[7]. Cathode materials are the positive electrodes of batteries and host the mobile ions (in this case lithium). The mobile ions are the ions that are removed from the structure when the battery is being charged and when the battery discharges (depletes) the ions are inserted back into the structure[8]. Current cathode materials are not only expensive to produce but also have some safety issues like overheating and short-circuiting associated with them – challenges we want to overcome.
Compared with other cathode materials, LiFePO4 has the advantage of being environmentally friendly, meaning there are no toxic materials presents, relatively cost efficient (no expensive metals/rare earth metals needed to synthesise the material) and is structurally and thermally stable. This means that the structure does not collapse with the removal of the mobile ions and the structure prevents the battery from overcharging as well as overheating, making this material safer to use[9]. However, one of the main disadvantages of LiFePO4 is the electrochemical performance such as the ionic (movement of ions through the crystal lattice)[10] and electronic (the ability to conduct or resist electric current), which are both important properties for cathode material. The mobile ions are restricted to movement through a 1-dimensional channel. Overcoming these problems has been the main focus for most research groups[11].
In my research, the electrochemical performance can be improved by doping with a selection of different metal ions. Inserting small amounts of metal ions into the structure can improve the battery performance differently depending on the metal. For example, nickel improves the stability of the structure and enhances the movement of lithium through the structure; copper improves the conductivity and improves the reversibility of the lithium ions in the structure; and manganese improves the reversibility as well as the stability of the structure.
Exploring materials through multiple techniques and collaborative efforts
To fully understand my material, it is very important to understand how the structure changes with small changes in my synthetic method and it is the collaborative effort between the Chemistry and Physics Departments at my university – the University of the Witwatersrand – which makes this possible, and through access to world class synchrotron sources to utilise the benefits of synchrotron data to further characterise my materials. Selected samples were sent to the Synchrotron source at Brookhaven National Laboratory (NSLS-II) in the USA and to Diamond Light Source, the UK’s national synchrotron (Diamond), with access to Diamond provided by the GCRF START grant. We have obtained data from synchrotron X-ray diffraction and total scattering, as well as X-ray absorption spectroscopy. Having remote access to state-of-the-art synchrotron equipment in this COVID-19 travel restricted world is heaven-sent as the research can continue even when no travelling is allowed.
To thoroughly characterise my synthesised materials, I have made use of our lab-based diffractometers in the Chemistry department at Wits as well as the Mössbauer spectrometer and the Raman spectrometer in the Physics department. Mössbauer spectroscopy and Raman spectroscopy are very useful for identifying crystalline (presence of long-range order of the atoms – regular arrangement of atoms over a longer distance) as well as amorphous (only the presence of short-range order of atoms – regular arrangement of atoms but only over a short distance) species in my samples. It is important to identify all the crystalline and amorphous species in the sample as impurities can occur in both forms and could negatively affect the battery performance.
Mössbauer spectroscopy is also useful for identifying the different local iron (Fe) environments present in my sample and to determine the form of iron – (oxidation state – Fe2+ is the desired state in my samples). Raman spectroscopy aids as a structural fingerprint that can be used to determine the identity of one’s material and is also useful in identifying any impurities present that the lab diffractometers could not detect due poor detection limits or due to the phase being amorphous. Synthesised samples can have a mixture of the desired product as well as impurities. There could be multiple sources for the formation of impurities but the most common causes can be either synthesis related (impurities that are formed due to a specific synthetic route) or impurities formed due to sensitivity to air (being exposed to air could cause some small changes like a change in oxidation state of a metal). Impurities can block the channel and subsequently the movement of the mobile ion and negatively affect the performance of the material and the battery.
Michelle Nyoni’s research: investigating Lithium vanadium phosphate for improved battery cathode materials
“Michelle Nyoni is a lady who, against many odds, is striving to obtain her PhD in energy materials. Impacted currently by Covid19 travel restrictions, Michelle is normally an ‘out-of-seat’ student who works full time at Chinhoyi University of Technology (CUT) in Zimbabwe and comes to South Africa for laboratory experiments at the University of the Witwatersrand during her teaching breaks as facilities for her research topic at CUT are limited. She has worked hard during her visits here to gather sufficient data so that she can process it when she returns home and brings with her a great deal of positivity and energy on each visit.” – Prof. Caren Billing, Prof. Caren Billing, University of the Witwatersrand.
While working in the farming sector in my home country of Zimbabwe, I realised that we are blessed with abundant renewable sources of energy – wind and solar – yet hindered by the challenge of how to store this energy effectively. This is where the subject of batteries came into my life and where my current PhD research area fits in. I am investigating lithium vanadium phosphates as cathode materials for lithium-ion batteries. I began my PhD studies part-time, in 2017, at the University of the Witwatersrand in South Africa under the supervision of Prof. Caren Billing and Prof. Dave Billing, while working as a Chemistry Lecturer at CUT in Zimbabwe.
The inspiration for my research is the fact that South Africa is one of the biggest vanadium producers in the world and Zimbabwe is one of the biggest lithium producers in the world. Therefore, if the raw materials are locally available it will hopefully mean reduced cost of battery production. My research is directly linked to Sustainable Development Goal (SDG) 7 concerning affordable and clean energy but by contributing to SDG 7, my research also contributes to achieving SDGs 1-6 and 8-9.
The aim of my research is to do much of the material characterisation by focussing on understanding what is happening at the atomic level, asking questions like: what is happening with the structure and the material? Lithium vanadium phosphate materials have been made but is this synthesis method reproducible? Does it work for upscaling to commercial levels? How do slight changes within the synthesis (preparation method) affect the material? How does adding a dopant manipulate the electrochemical properties of my material?
Lithium vanadium phosphates are potentially effective because their various properties are attractive – they have a high thermodynamic and kinetic stability, and studies have shown they possess the potential to have very good electrochemical properties, which means they will have high specific energy, high working voltage, and good cycle stability. Normally, as batteries get older, the cycling gets poorer and poorer but lithium vanadium phosphate materials have good cycle stability and a lower price tag so they are not as expensive as some of the alternative materials that can be used. There are also other advantages. Lithium vanadium phosphate materials provide improved safety, phosphates are more environmentally friendly than some other materials[12], and the vanadium contributes to the energy density as well as the voltage of the cells – in fact, our Lithium vanadium phosphate materials can reach voltages of over 4 volts! A key application is in electric vehicles which will benefit from increased length of travel due to the cycle stability of Lithium vanadium phosphate materials and the higher specific energy density, amongst other improvements due to the advantages described above.
The cathode material determines the voltage and capacity of a battery and the cathode in a lithium-ion battery is the positive electrode, which is normally a metal oxide that is responsible for being the source of lithium ions that carry the electric current when a battery is in use (discharging)[13]. There are various components that contribute to the cost of the battery with the cathode material within the battery usually one of the biggest costs, along with the separator[14]. The lithium vanadium phosphate materials that I am working with are cathode materials, therefore if we can source these locally within the SADC region of Africa, which includes 15 member states, this would make them a lot more affordable and accessible – which is the goal I am driving at.
Cutting edge techniques to determine material characterisation and impact
The techniques I will use in my PhD studies aim to test the lithium vanadium phosphate materials in depth so that I can contribute to research that is already available to help find viable products that can be used in Africa. Techniques include powder X-ray diffraction for the phase identification and Raman spectroscopy to enable me to determine the structural finger-print to ensure I am making the same product each time so that when I do change a parameter, the resulting effect will be clear. The GCRF START grant enables us to use the Diamond synchrotron for variable temperature experiments. Therefore, I would want to look out for how the material changes when we vary the temperature. I would also use XAFS– X-ray absorption fine structure spectroscopy at the B18 beamline at Diamond to study the changes in the neighbourhoods of particular atoms.
Another technique is transition electron microscopy – the determination of particle size and the distribution of those particles as well as the general morphology of the fine particles within the material. Additionally, I want to use STA – Simultaneous Thermal Analysis – to look at the thermal stability to ask a series of questions: how stable is my material and what happens under temperature changes? Does it break down or decompose? How does this effect the overall electrochemical properties because when we use these batteries they will heat up? What is the impact, for example, if I were to leave my phone device with a battery using these materials in the sun – how would the warmth of the sun affect it? Would the structure and performance be impacted? Therefore, I would do extensive electrochemical testing which includes cyclic voltammetry and electrochemical impedance tests, amongst others, to ensure the batteries with these materials are viable in the varied environmental conditions found across Africa, including very warm environments.
the Chinhoyi University of Technology (CUT), Zimbabwe. ©Diamond Light Source
The GCRF START grant: a bridge to sustainable growth and life-changing possibility
“Being part of the GCRF START collaboration has certainly taken our work in energy materials in South Africa to the next level!”
Prof. Caren Billing, University of the Witwatersrand
Many of the Group’s research projects are now at the point where data has been measured and obtained and the next learning curve of how to process the data is underway. Progress has been made, Prof. Caren Billing reports, which, without funding from the GCRF START Grant, would have been an even larger hurdle to overcome.The resultis cutting-edge science and capacity building, knowledge exchange and access to the latest techniques and technology, and a new generation of gifted scientists working towards the shared vision of developing novel, green and affordable energy solutions to inspire life-changing possibility in Africa and beyond.
“This is where the GCRF START grant comes in,” says Prof. David Billing, “it provides that bridge. Yes, there’s a skills gap here in Africa but for me that gap is possibly smaller than others; as long as we are staying current on the XRD side we can transition easily and tackle the more challenging newer techniques – there’s a whole suite of them but that will grow – and START gets us there! This is also what you need to get to the higher impact journals; it also to gets us closer to current answers and future possibilities rather than just ‘the best we can do’ with 30-year-old technology.”
“In terms of energy solutions, take the scenario of load shedding (electricity cuts) which poses a huge challenge across countries in Africa. The thought of being able to go off grid is vital. If you think about a rural village which is cooking using wood or charcoal and lighting in the form of paraffin or candles – this is energy poverty. If you can find a cheap, clean, sustainable source of energy to replace these – that would be life changing!”
GCRF START Co-I, Prof. David Billing, Professor in the School of Chemistry and Co-PI of the Energy Materials Research Group at Wits.
Energy Materials Research Group at the University of the Witwatersrand, South Africa. ©Diamond Light Source
South Africa; and Co-PI of the Energy Materials Research Group and GCRF START Co-I. ©Diamond Light Source
Footnotes
[1] United Nations Sustainable Development Goals: Energy for Sustainable Development, https://sdgs.un.org/topics/energy
[2] Introducing small amounts of other metal ions into the structure during synthesis without changing the structure of the material
[3] Prof. David Billing is also Assistant Dean in the Faculty of Science at the University of the Witwatersrand, South Africa.
[4] https://www.un.org/sustainabledevelopment/energy/
[5] https://www.gcis.gov.za/sites/default/files/docs/resourcecentre/yearbook/yb1919-16-Mineral-Resources.pdf
[6] An easy way to picture this is in terms of ores. Generally, an ore will contain a mixture of minerals and hence is not ‘phase pure’.
[7] Information on the natural occurring triphylite (mineral data): https://www.mindat.org/min-4020.html as well as an electrochemical comparison: https://www.sciencedirect.com/science/article/pii/S0378775301007273?casa_token=zcBRvEsb5SMAAAAA:Lrk2oIjOY-9W73OBWasFIrxQaP7mhWNfQ4HrYjzx7Ib_r95Pq4ix8eORm0IBm29G-izoI18
[8] How Lithium batteries work in: https://www.explainthatstuff.com/how-lithium-ion-batteries-work.html
[9] Advantages and disadvantages of Lithium-iron-phosphate v lithium ion: https://blog.epectec.com/lithium-iron-phosphate-vs-lithium-ion-differences-and-advantages
[10] See Figure 1. How the lithium ions move in a battery in: https://www.spectroscopyonline.com/view/techniques-raman-analysis-lithium-ion-batteries; see also: The channels through which lithium has to move in LiFePO4 in the paper: Yi, T., Li, X., Liu, H. et al. Recent developments in the doping and surface modification of LiFePO4 as cathode material for power lithium ion battery. Ionics 18, 529–539 (2012). https://doi.org/10.1007/s11581-012-0695-y
[11] Jingkun Li, Zi-Feng. Past and Present of LiFePO4: From Fundamental Research to Industrial Applications. Chem. Volume 5, Issue 1, 10 January 2019, Pages 3-6 (2019), Elsevier. https://doi.org/10.1016/j.chempr.2018.12.012;
V.S.L. Satyavani,A. Srinivas Kumar,P.S.V. Subba Rao.Methods of synthesis and performance improvement of lithium iron phosphate for high rate Li-ion batteries: A review. Engineering Science and Technology 19, Issue 1, March 2016, Pages 178-188. Elsevier. https://doi.org/10.1016/j.jestch.2015.06.002;
Yi, T., Li, X., Liu, H. et al. Recent developments in the doping and surface modification of LiFePO4 as cathode material for power lithium ion battery. Ionics 18, 529–539 (2012). https://doi.org/10.1007/s11581-012-0695-y
[12] Hameed, S.A., Reddy, M.V., Sakar, N., Chowdari, B.V.R. & Vittal, J.J.; Royal Society of Chemistry Advances 2015, 5, 60630-60637
[13] See: ‘The four components of a Lithium battery’: https://www.samsungsdi.com/column/technology/detail/55272.html?pageIndex=1&idx=55272&brdCode=001&listType=list&searchKeyword=
[14] See: Figure 3. ‘Total material costs of all 10 considered cell chemistries plus Panasonic NCA Use Case differentiated in combined CAM cost, anode cost, and secondary material costs’ in: Wentker, M.; Greenwood, M.; Leker, J. A Bottom-Up Approach to Lithium-Ion Battery Cost Modeling with a Focus on Cathode Active Materials. Energies 2019, 12, 504. https://doi.org/10.3390/en12030504